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HOME  UNIVERSITY  LIBRARY 
OF  MODERN  KNOWLEDGE 

No.  90 

Editors : 

HERBERT    FISHER,  M.A.,  F.B.A. 
PROF.    GILBERT    MURRAY,    LiTT.D., 

LL.D.,  F.B.A. 

PROF.  J.  ARTHUR    THOMSON,  M.A. 
PROF.  WILLIAM    T.  BREWSTER,  M.A. 


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SCIENCE 
Already  Published 

ANTHROPOLOGY By  R.  R.  MARETT 

AN    INTRODUCTION   TO 

SCIENCE By  J.  ARTHUR  THOMSON 

EVOLUTION By  J.  ARTHUR  THOMSOI-J  and 

PATRICK  GEDDES 

SEX By  PATRICK  GEDDHS  and 

J.  ARTHUR  THOMSON 

THE  ANIMAL  WORLD By<F.  W.  GAMBLE 

INTRODUCTION     TO     MATHE- 
MATICS      By  A.  N.  WHITEHEAD 

ASTRONOMY By  A.  R.  HINKS 

PSYCHICAL  RESEARCH  .    .   .   .  By  W.  F.  BARRETT 
THE   EVOLUTION   OF   PLANTS  By  D.  H.  SCOTT 
CRIME   AND   INSANITY  .    .    .    .   By  C.  A.  MERCIER 
MATTER  AND   ENERGY  ....   By  F.  SODDY 

PSYCHOLOGY By  W.  McDouGALL 

PRINCIPLES    OF    PHYSIOLOGY  By  J.  G.  MCKENDRICK 
THE   MAKING  OF  THE  EARTH  By  J.  W.  GREGORY 

ELECTRICITY By  GISBERT  KAPP 

MAN:     A    HISTORY     OF    THE 

HUMAN  BODY By  A. KEITH 

THE     ORIGIN     AND     NATURE 

OF   LIFE By  BENJAMIN  MOORE 

DISEASE  AND  ITS  CAUSES  .   .  By  W.  T.  COUNCILMAN 

PLANT  LIFE By  J.  B.  FARMER 

NERVES By  D.  F.  HARRIS 

CHEMISTRY By  RAPHAEL  MELDOLA 

Future  Issues 

THE  CARE  OF  CHILDREN   ...  By  R.  A.  BENSON,  M.D. 
THE   MINERAL  WORLD   ....   By  SIR  T.  H.  HOLLAND 


CHEMISTRY 


BY 


RAPHAEL   MELDOLA 

D.SC.,  LL.D.,  F.R.S. 

PROFESSOR    OF    CHEMISTRY    IN    THE    FINSBURY    TECHNICAL 

COLLEGE  }    AUTHOR  OF  "THE  CHEMICAL  SYNTHESIS 

OF  VITAL  PRODUCTS,"    ETC. 


NEW   YORK 
HENRY   HOLT  AND   COMPANY 

LONDON 
WILLIAMS   AND    NORGATE 


CONTENTS 

CHAFTKK  »AGE 

INTRODUCTORY          .  7 

I  THE  SCOPE  OF  CHEMISTRY — THB  NATURE 
OF  CHEMICAL  CHANGE  —  CHEMISTRY  AN 
EXPERIMENTAL  SCIENCE  .  .  .  16 
II  CHEMICAL  COMBINATION  AND  MECHANICAL 
MIXTURE — AIR  A  MIXTURE  AND  NOT  A 
COMPOUND  —  PHYSICAL  SEPARATION  OF 
THE  COMPONENTS  OF  AIR  .  .  .  41 

III  CHEMICAL  CHANGE  IN   ITS    QUANTITATIVE 

ASPECT — THE  DEFINITENESS  OF  CHEMICAL 
CHANGE — THE  CONSERVATION  OF  MASS — 
WATER  A  CHEMICAL  COMPOUND  .  .  64 

IV  ELEMENTARY    AND    COMPOUND    MATTER — 

THE  CHEMICAL  ELEMENTS — METALS  AND 
NON-METALS          .         .         .         .         .91 
V    CHEMICAL  EQUIVALENCE — ELECTRO -CHEMI- 
CAL    EQUIVALENCE  —  MULTIPLE     AND 
RECIPROCAL  EQUIVALENCE — THE  ATOMIC 

THEORY 115 

VI  SYMBOLS  AND  NOTATION — ATOMS  AND 
MOLECULES  —  ATOMIC  AND  MOLECULAR 
WEIGHTS  —  THE  DEFINITENESS  OF 
CHEMICAL  COMBINATION  BY  VOLUME 
— THE  HYPOTHESIS  OF  AVOGADRO  .  137 
VII  THE  NUMBER  OF  ATOMS  CONTAINED  IN 
A  MOLECULE — DISSOCIATION  AND  ASSO- 
CIATION —  AUXILIARY  METHODS  FOR 
DETERMINING  ATOMIC  AND  MOLECULAR 
WEIGHTS — THE  LAW  OF  DULONG  AND 
PETIT — ATOMIC  AGGREGATES  IN  SOLUTION  163 


6  CONTENTS 

CHAPTER  PAGE 

VIII  DETERMINATION  OF  THE  RELATIVE  WEIGHTS 
OF  THE  ATOMS  —  THE  ISOLATION  OF 
DEFINITE  SUBSTANCES — CHEMISTRY  AS 
AN  EXACT  SCIENCE — THE  STANDARD  OF 
ATOMIC  WEIGHTS  —  CHEMICAL  ARITH- 
METIC— VOLUMETRIC  RELATIONSHIPS  .  183 
IX  VALENCY  —  CHEMICAL  STRUCTURE  —  THE 
CHEMISTRY  OF  CARBON — STEREOCHEMIS- 
TRY .  .  .  .  .  .  .204 

X    THE    PERIODIC    CLASSIFICATION    OF     THE 

ELEMENTS — CONCLUSION         .         .         .  228 

BIBLIOGRAPHY 249 

INDEX     .....  253 


CHEMISTRY 


INTRODUCTORY 

THE  history  of  civilization  reveals  the  fact 
that  all  highly  developed  nations  in  the  course 
of  their  evolution  have  passed  through  phases 
characterised  by  the  culmination  of  various 
human  activities,  physical  and  intellectual. 
Not  that  it  is  implied  by  this  statement  that 
the  manifestation  of  extreme  activity  of  a 
particular  kind  at  one  period  was  accompanied 
by  a  decline,  or  was  developed  at  the  expense 
of  all  other  forms  of  activity.  The  lesson  of 
history  is  that,  concurrently  with  the  general 
national  activity,  certain  ages  have  witnessed 
special  activities  or  have  attained  particular 
maxima  of  development  which  have  served 
to  stamp  the  age  with  some  general  charac- 
teristic. Thus,  there  was  an  age  of  Philosophy 
in  ancient  Greece,  of  Militarism  in  ancient 
Rome,  of  Sacred  Art  in  mediaeval  Italy,  and 
of  Dramatic  Poetry  in  England  during  the 
Elizabethan  period.  The  influence  of  such 
epochs  has  extended  perceptibly  or  imper- 


ceptibly  from  a  remote  past  down  to  the 
present  time  ;  the  recognition  of  this  influence 
is  embodied  in  the  familiar  adage  that  we 
are  the  heirs  of  all  the  ages.  The  special 
activity  of  the  present  time,  Science,  is  one 
that  we  believe  is  destined  to  influence  the 
future  more  profoundly  than  any  of  those 
activities  which  reached  their  culminating 
points  in  former  ages. 

In  stating  that  we  are  now  living  in  an 
age  of  Science,  it  is  meant  that  we  are  getting 
into  closer  communion  with  Nature  than  has 
hitherto  been  possible.  From  the  time  when 
man  became  an  observing  and  thinking  being, 
he  must  have  been  impressed  by  natural 
phenomena ;  but  at  no  former  period,  so  far 
as  history  has  preserved  records,  has  there 
been  such  intense  activity  in  the  questioning 
of  Nature — in  the  systematized  observation 
of  facts,  and  in  the  endeavour  to  arrive  at  a 
knowledge  of  causes.  It  may  be  said  that, 
among  the  more  advanced  nations,  mankind 
is  gradually  beginning  to  grasp  that  great 
truth  which  in  former  ages  was  realized  only 
by  a  few  specially  gifted  individuals — the  truth 
that  the  human  race,  although  the  intellectual 
crown  and  summit  of  terrestrial  life,  is  not 
detached  from  and  independent  of  its 
surroundings.  The  anthropocentric  notions 


INTRODUCTORY  9 

which  dominated  thought  in  early  times  are 
slowly  being  replaced  by  that  broader  view 
which  makes  man  a  part  of  Nature — an 
organism  adapted  to  his  environment  just 
like  any  other  organism,  but  having  the 
supreme  advantage  of  practically  unlimited 
adaptability  by  virtue  of  his  intellectual 
development.  It  is  now  beginning  to  be 
perceived  that  this  power  of  adaptation  is 
synonymous  with  a  knowledge  of  Nature's 
methods — in  other  words,  that  the  present 
well-being  and  the  future  progress  of  the 
human  race  is  dependent  upon  the  develop- 
ment of  Natural  Science. 

The  recognition  of  the  principle  that  man's 
dominion  is  inseparably  bound  up  with 
scientific  progress  is  embodied  in  Tennyson's 
lines  : — "  The  crowning  race  ;  Of  those  that 
eye  to  eye  shall  look  on  knowledge  ;  Under 
whose  command  is  Earth  and  Earth's  ;  And 
in  their  hand  is  Nature  like  an  open  book." 

This  recognition  has  been  brought  about  in 
modern  times  by  the  labours  of  those  who 
have  devoted  and  are  devoting  their  lives  to 
the  study  of  Nature  at  first  hand.  It  is  the 
active  army  of  original  investigators  who, 
in  the  first  place,  have  become  cognizant  of 
the  supreme  importance  of  their  work  to  the 
present  and  future  welfare  of  the  race.  The 


10  CHEMISTRY 

realization  of  the  truth  that  Nature  is  to  the 
earnest  student  "  an  open  book  "  has  become 
the  trumpet  call  of  the  present  age.  The 
worker  in  the  domain  of  Science  is  prompted 
by  the  knowledge  that  his  results,  directly 
or  indirectly,  immediately  or  prospectively, 
may  be  utilized  for  the  benefit  of  humanity. 
His  achievements,  although  strictly  humani- 
tarian in  their  ultimate  bearing,  cannot, 
however,  be  weighed  and  measured  by  a 
narrowly  practical  standard.  The  level  of 
natural  knowledge  which  has  now  been  reached, 
and  which  is  annually  being  raised,  is  the 
result  of  patient  and  laborious  research,  often 
extending  over  many  years,  sometimes  over 
a  lifetime.  But  only  a  small  proportion  of 
the  work  accomplished  is  of  immediate 
utility  ;  and  that  which  is  obviously  useful 
to  man  is  but  the  final  stage  of  a  long  series 
of  antecedent  gropings  after  truth.  The 
popular  appreciation  of  Science,  to  be  of  real 
value  to  the  nation,  should,  therefore,  be 
independent  of  the  spirit  of  narrow  utilitarian- 
ism, for  no  investigator  who  enters  upon  a 
definite  line  of  work  can  foresee  when  or  how 
his  results  may  become  of  practical  value, 
or  whether  they  will  ever  lead  to  any  practical 
applications.  If  the  progress  of  the  nation 
is  dependent — as  we  are  now  beginning  to 


INTRODUCTORY  11 

realize — upon  its  general  appreciation  of 
Science,  that  appreciation  must  be  of  the 
highest  and  broadest  character — it  is  Science 
in  the  abstract,  and  not  purely  utilitarian 
concrete  knowledge,  which  must  be  raised  to 
the  level  of  one  of  the  most  exalted  branches 
of  human  culture. 

The  modern  awakening  of  the  spirit  of 
scientific  inquiry  has  resulted  in  an  activity 
which  is  in  itself  responsible  in  some  measure 
for  the  slow  progress  towards  the  attainment 
of  that  high  standard  of  popular  scientific 
culture  which  we  desire  to  see  established. 
The  active  workers  are  a  numerous  and  ever 
growing  body,  and  the  boundaries  of  knowledge 
are  being  extended  with  such  rapidity  in  every 
direction  that  the  educated  layman  who  can 
follow  with  intelligence  the  various  develop- 
ments of  Literature  or  of  Art  finds  himself 
unable  to  cope  with  the  progress  of  Science. 
Nor  is  this  surprising  when  we  find  that  even 
the  workers  themselves,  having  necessarily 
to  specialize  in  order  to  achieve  results  of 
value,  are  unable  to  keep  pace  with  the 
progress  of  discovery  in  domains  outside  their 
own  field  of  research.  Moreover,  the  constant 
discovery  of  new  facts  and  principles,  and  the 
concurrent  revision  or  extension  of  scientific 
doctrine  is  apt  to  discourage  the  would-be 


12  CHEMISTRY 

learner  who,  without  any  special  scientific 
training,  has  had  his  mind  deprived  of  plas- 
ticity by  an  inelastic  education  in  subjects  for 
which  the  materials  are  gathered  entirely  from 
books  and  not  directly  from  Nature's  records. 

Another  obstacle  in  the  way  of  the  general 
diffusion  of  scientific  culture  is  the  technical 
language  which  every  branch  of  science  has 
found  it  necessary  to  invent  in  order  to  give 
precision  to  the  description  of  new  facts,  and 
for  the  formulation  of  new  principles.  But, 
while  admitting  that  the  technicalities  of 
modern  scientific  language  from  the  popular 
point  of  view  interpose  difficulties,  it  must 
be  borne  in  mind  that  for  the  actual  workers 
they  are  labour-saving  contrivances.  Although 
the  terminology  may  appear  formidable 
to  the  uninstructed,  it  must  not  be  for- 
gotten that  every  term  and  every  symbol 
corresponds  with  some  natural  reality,  or 
with  what  according  to  existing  knowledge 
is  believed  to  be  a  reality.  The  reality  is 
generally  capable  of  being  expressed  in 
simpler  terms  than  would  appear  from  its 
symbolical  expression  ;  the  underlying  idea 
is  generally  less  complex  than  the  language 
which  has  been  found  necessary  to  define  it 
with  scientific  precision. 

But,  apart  from  all  such  difficulties,  in  view 


INTRODUCTORY  13 

of  the  daily  increasing  importance  of  Science 
as  a  prime  factor  of  national  development, 
the  educated  layman  can  no  longer  afford  to 
ignore  the  achievements  of  that  great  inter- 
national army  which  is  waging  perpetual 
warfare  against  ignorance  of  Nature's  methods. 
In  this  quest  for  knowledge,  there  is  no  dis- 
tinction of  race,  or  creed,  or  country — all 
workers  are  co-operating  for  the  general 
cause  ;  a  truth  wrested  from  Nature  becomes 
the  common  property  of  mankind.  Such 
truths  cannot  be  lightly  set  aside,  or  crushed 
out  of  existence  by  the  older  learning  ;  they 
are  revelations  to  man  as  distinct,  as  eternal, 
and  as  far  reaching  in  their  consequences  as 
any  proclaimed  by  the  seers  and  prophets  of 
former  ages. 

Granting,  therefore,  that  the  reader  wishes 
to  be  put  in  possession  of  the  existing  state 
of  scientific  knowledge,  it  must  at  the 
outset  be  realized  that  Science  never  pauses 
on  her  onward  march ;  there  is  no  "  existing 
state  "  of  knowledge  in  the  sense  of  finality. 
She  has  no  dogmatic  creed  to  proclaim ;  she 
is  aware  of  her  fallibility ;  and  her  strength 
lies  in  her  knowledge  that  it  is  Nature  which 
is  infallible,  and  man  but  an  interpreter  with 
limited  power  of  observation  and  reasoning. 
The  ground  covered  by  Science  is,  moreover, 


14  CHEMISTRY 

so  vast  that  it  must  also  be  recognized  that, 
for  the  practical  purposes  of  study  and  re- 
search, sub-division  into  departments  is  abso- 
lutely necessary.  Not  that  these  sub-divisions 
are  representative  of  any  natural  reality ; 
they  are  expressive  rather  of  the  imperfect 
state  of  our  knowledge.  In  view  of  the 
limitations  of  human  faculty,  it  is  both  necess- 
ary and  expedient  that  the  worker  should 
confine  himself  to  some  particular  department ; 
but,  in  accepting  this  principle  as  a  matter 
of  convenience,  the  student  must  not  commit 
himself  to  the  belief  that  this  sub-division 
indicates  a  want  of  unity  in  Nature.  On  the 
contrary,  the  most  advanced  thinkers  have 
come  to  believe  in  the  unity  of  Nature  and 
to  recognize  that  the  ideal  towards  which 
research  is  tending  is  the  unification  of  know- 
ledge into  one  general  Science  or  system  of 
Philosophy.  There  may  be  work  for  countless 
generations  before  this  ideal  is  reached,  but 
even  now  there  are  indications  in  every 
direction  that  natural  knowledge  cannot  be 
confined  in  water-tight  compartments  ;  the 
barriers,  confessedly  artificial,  are  being  broken 
down,  and  the  inter-relations  between  the 
various  sciences  are  becoming  both  more 
numerous  and  more  intimate  with  the  progress 
of  discovery.  The  tendency  towards  coales- 


INTRODUCTORY  15 

cence  is  shown  by  the  creation  in  modern 
times  of  such  subjects  as  thermodynamics, 
astrophysics,  chemical  physics  and  physical 
chemistry,  electrochemistry,  thermochemis- 
try, biochemistry,  and  biophysics. 


CHAPTER  I 

THE  SCOPE  OF  CHEMISTRY — THE  NATURE  OF 
CHEMICAL  CHANGE CHEMISTRY  AN  EX- 
PERIMENTAL SCIENCE 

The  Scope  of  Chemistry. — The  Science  of 
Chemistry,  as  is,  no  doubt,  already  known  to 
the  reader  in  a  general  way,  is  essentially  a 
materialistic  science  in  so  far  as  it  deals  with 
matter.  It  belongs  to  a  division  known  as  the 
physical  sciences,  a  designation  applied  in 
order  to  distinguish  such  subjects  from  those 
which,  like  Zoology,  Botany,  Physiology,  etc., 
deal  with  life,  and  which  are,  therefore, 
grouped  as  the  biological  sciences.  This 
classification  is  convenient  as  representing 
the  existing  state  of  knowledge ;  whether 
such  sub-division  corresponds  with  some 
underlying  fundamental  reality  is  a  debate- 
able  question  concerning  which  no  dogmatic 
pronouncement  can  at  present  be  made. 
But  as  with  all  attempts  at  rigid  classification, 
so  here  it  will  be  found  that  no  absolute 
barrier  can  be  erected  between  the  two 

16 


SCOPE   OF   CHEMISTRY  17 

groups.  Living  matter,  whether  lowly  or 
highly  organized,  is  as  subject  to  physical 
and  chemical  conditions  as  non-living  matter. 
The  Physics  and  Chemistry  of  the  living 
organism  are  no  longer  regarded  as  impene- 
trable mysteries  beyond  the  scope  of  legitimate 
scientific  investigation.  Modern  Chemistry 
does  not  recognize  that  rigid  definition  which 
in  former  times  restricted  its  scope  to  "  dead 
matter  "  ;  in  that  borderland  between  the 
two  main  groups  of  sciences,  there  is  now  at 
work  a  new  school  of  investigators  who  are 
attacking  the  mysteries  of  vital  chemistry 
in  the  same  spirit  as  that  which  has  prompted 
research  into  every  other  department  of 
science.  It  is  realized  that  the  living  organism 
has  solved  chemical  problems  which  we  have 
as  yet  been  unable  to  approach  by  our  known 
methods — but  this  is  regarded  as  an  incentive, 
and  no  longer  as  a  deterrent,  to  further 
inquiry.  On  the  other  side,  the  biologist 
deals  with  living  organisms  not  only  from 
the  point  of  view  of  classification,  distribution, 
bionomics,  evolution,  etc.,  but  he  also  con- 
cerns himself  with  their  physical  and  chemical 
activities,  with  the  inner  mechanism  of  the 
life  processes.  Physiology,  in  the  broad 
sense,  has  now  become  the  meeting-ground 
of  the  physical  and  biological  sciences. 


18  CHEMISTRY 

In  placing  Chemistry  in  the  front  rank  as  a 
science  concerned  with  matter,  we  thereby 
associate  it  with  other  sciences  which  also 
deal  with  matter,  and  especially  with  Physics, 
its  nearest  and  natural  ally.  In  view  of  what 
has  been  said  concerning  the  conventionality 
of  our  schemes  of  classification,  it  is  perhaps 
unnecessary  to  insist  upon  the  impossibility 
of  drawing  a  sharp  line  of  demarcation 
between  Chemistry  and  Physics.  No  other 
branches  of  science  furnish  such  numerous  and 
such  striking  examples  of  interpenetration. 
Physical  methods  of  studying  the  properties 
of  matter  are  used  by  chemists,  and  chemical 
methods  are  being  adopted  by  physicists. 
All  that  can  be  said  definitely  is  that  those 
general  properties  of  matter,  such  as  gravity, 
which  are  independent  of  the  specific  nature 
or  composition  of  the  substance,  belong  more 
exclusively  to  the  domain  of  the  physicist. 
But  even  in  this  case,  the  chemist  is  dependent 
upon  gravitational  effect  for  the  determination 
of  weight — the  most  important  of  known 
methods  for  dealing  with  matter  quantita- 
tively. 

Whenever  we  pass  from  the  general  to  the 
special,  and  consider  in  detail  the  changes 
in  matter  brought  about  by  the  action  of 
physical  forces,  it  will  be  seen  that  the  nature 


SCOPE   OF   CHEMISTRY  19 

or  amount  of  change  is  generally  influenced 
by  the  inherent  properties  of  the  substance, 
or,  in  other  words,  by  those  specific  characters 
which  are  as  truly  chemical  as  physical.  Thus, 
heating  and  cooling  cause  bodies  to  expand 
or  contract,  a  change  which,  in  general  terms, 
may  be  said  to  be  physical.  But  in  this 
case  it  cannot  be  said  that  the  change  is 
independent  of  the  inherent  nature  of  the 
substance ;  for,  when  the  expansion  or 
contraction  is  measured  quantitatively,  it  is 
found  to  be  a  specific  character,  and  therefore 
of  chemical  significance.  Again,  a  ray  of 
light  is  bent  or  refracted  in  passing  from  one 
medium  to  another  of  different  density ; 
here,  also,  the  general  effect  is  said  to  be 
physical.  But  the  actual  amount  of  bending, 
as  measured  quantitatively,  is  found  to  be 
determined  by  the  specific  character  or 
composition  of  the  medium  ;  and  the  relation- 
ship is  so  close  that  in  certain  classes  of 
cases  the  measurement  of  this  quantity  gives 
information  to  the  chemist  concerning  the 
nature  of  the  substance.  One  of  the  most 
striking  illustrations  of  the  association  of 
Physics  with  Chemistry  is  furnished  by  the 
results  obtained  by  the  study  of  the  action 
of  electricity  upon  matter  in  the  gaseous 
state,  a  field  of  research  which  has  led  in 


20  CHEMISTRY 

recent  times  to  suggestions  of  fundamental 
importance  concerning  the  constitution  of 
matter.  It  is  evident  that  such  questions 
are  of  equal  significance  to  Physics  and  to 
Chemistry ;  but  the  modern  developments 
in  this  direction  are  as  yet  hardly  mature 
enough  to  incorporate  with  the  established 
body  of  chemical  doctrine.  As  an  earlier 
illustration,  however,  nothing  is  more  instruc- 
tive than  the  history  of  the  development 
of  spectroscopy.  A  gas  or  vapour  when 
raised  to  a  sufficiently  high  temperature  to 
glow  or  become  self  luminous  emits  light 
which  appears  to  have  colour.  In  purely 
physical  terms,  this  is  explained  by  the 
statement  that  the  colour  is  the  result  of  the 
impression  produced  upon  the  eye  by  ether 
waves  of  particular  lengths  or  oscillation 
frequencies.  But,  here  again,  when  we  go 
into  details,  and  analyse  the  light  emitted 
by  such  glowing  gases,  we  find  that  the 
particular  kind  of  radiation  is  a  specific 
character  of  the  radiating  matter — sufficiently 
specific  for  diagnostic  purposes,  so  that 
particular  kinds  of  matter  can  be  identified, 
whether  here,  or  in  the  sun,  or  in  the  most 
distant  stars  and  nebulae.  The  chemist  and 
physicist  and  astronomer  here  join  hands  ; 
the  science  based  upon  this  property  of 


CHEMICAL    CHANGE  21 

matter  is  both  astro-chemical  and  astro- 
physical. 

It  is  obvious  from  such  considerations  that 
Chemistry  is  all-embracing  in  its  scope ;  its 
branches  ramify  in  every  direction,  and  its 
roots  underlie  the  most  diverse  departments 
of  science.  Since  it  concerns  itself  with  matter 
wherever  it  exists  and  can  be  brought  within 
the  ken  of  its  methods,  it  may  be  regarded  as 
the  common  meeting-ground  of  all  the  natural 
sciences.  From  the  purely  utilitarian  point 
of  view,  no  other  science  can  be  said  to  be 
more  intimately  bound  up  with  the  immediate 
welfare  of  man.  The  prime  needs  of  civilized 
nations — the  raising  of  crops  for  food-stuffs, 
the  utilization  of  fuel  for  the  generation  of 
power  or  for  warmth,  the  manufacture  of 
useful  products  from  raw  materials,  the 
production  of  the  multifarious  necessary 
materials  required  in  every-day  life,  all  depend 
at  one  stage  or  another  upon  a  knowledge  of 
the  principles  of  chemical  science  and  their 
practical  application. 

The  Nature  of  Chemical  Change. — Chemistry 
does  not  restrict  itself  to  the  study  of  matter 
simply  as  it  exists,  either  naturally  or  arti- 
ficially ;  it  is,  above  all,  the  science  which 
concerns  itself  with  the  changes  or  trans- 
formations of  matter  brought  about  by  the 


22  CHEMISTRY 

agency  of  physical  forces,  or  by  the  action  of 
one  form  of  matter  upon  another.  In  so  far 
as  the  mere  consideration  of  change  in  matter 
is  concerned,  Chemistry  and  Physics  overlap 
to  a  very  large  extent.  For  practical  purposes 
it  is,  however,  convenient  in  the  present  state 
of  knowledge  to  distinguish  between  physical 
and  chemical  change.  If,  under  the  influence 
of  physical  agencies  such  as  heat,  light, 
electricity,  etc.,  matter  undergoes  some  change 
which  is  not  permanent,  it  is  considered  to 
be  a  physical  change.  If  these  or  other 
agencies  bring  about  a  permanent  change  or 
transformation  of  one  kind  of  matter  into 
a  totally  different  substance,  the  action  is 
claimed  as  coming  within  the  province  of 
Chemistry.  Following  up  the  illustration 
previously  given  concerning  the  action  of 
heat,  a  bar  of  iron  is  longer  when  it  is  hot 
than  when  it  is  cold ;  but  the  change,  i.e., 
the  increase  in  length,  is  not  permanent, 
because  the  bar  regains  its  original  length  on 
cooling.  This  is  an  example  of  physical 
change,  and,  even  if  the  expansion  were 
permanent,  it  would  not  be  regarded  as  a 
chemical  change,  because  there  is  no  trans- 
formation of  material — the  bar  is  the  same 
substance,  iron,  both  hot  and  cold.  If  the 
iron  is  heated  to  a  very  high  temperature,  it 


CHEMICAL    CHANGE  23 

undergoes  further  changes,  first  becoming 
soft,  and  then,  as  the  temperature  rises, 
becoming  liquid,  so  that  it  can  be  poured  out 
of  the  containing  vessel.  These,  again,  are 
physical  changes  of  state,  because  the  material 
is  still  iron ;  by  the  process  of  fusion  its 
form  may  be  permanently  changed,  but  its 
substance  is  unaltered. 

Although  such  properties  of  matter  are 
described  as  "  physical,"  they  become  of 
chemical  significance  when  we  pass  from 
the  general  to  the  special — when  their  study 
is  made  comparative.  Thus  the  expansion 
of  iron  at  a  given  temperature  as  compared 
with  the  expansion  of  other  substances  ajt  the 
same  temperature,  or  the  temperature  at 
which  iron  melts,  i.e.,  passes  from  the  solid 
to  the  liquid  state,  may  be  claimed  as  physico- 
chemical  properties,  because  they  are  specific 
and  not  general ;  other  forms  of  matter  may 
or  may  not  have  the  same  degree  of  expansi- 
bility and  the  same  point  of  fusion.  When, 
however,  intensely  heated  iron  is  freely 
exposed  to  the  air,  or  is  plunged  into  water, 
it  undergoes  a  change  in  appearance ;  the 
surface  becomes  covered  with  an  incrustation 
or  scale  of  something  which  is  no  longer  iron. 
In  this  case,  the  change  is  permanent  so  long 
as  the  new  substance  remains  under  ordinary 


24  CHEMISTRY 

conditions  ;  iron  is  there,  but  in  a  disguised 
form,  and  can  only  be  recovered  from  the  new 
substance  by  violent  treatment,  such  as 
heating  to  an  extremely  high  temperature  in 
contact  with  coal  or  carbonaceous  matter. 
The  iron  in  this  case  has  undergone  a  chemical 
change  ;  it  has  become  transformed  into  a 
different  substance.  Even  without  the  appli- 
cation of  heat,  iron  on  exposure  to  air  and 
moisture,  as  every  one  knows,  becomes 
"  rusty,"  i.e.,  converted  into  a  reddish  sub- 
stance which,  like  the  "  scale  "  just  described, 
contains  iron,  but  is  no  longer  that  material ; 
in  rusting,  iron  loses  all  its  original  physical 
and  physico-chemical  properties.  It  is  changed 
in  hardness,  tenacity,  specific  gravity,  electric 
conductivity,  fusing  point,  magnetic  character, 
colour,  and  so  forth.  It  is  with  transforma- 
tions of  this  kind  that  Chemistry  is  especially 
concerned. 

Examples  illustrative  of  chemical  change 
might,  of  course,  be  multiplied  indefinitely  ; 
the  case  selected  on  account  of  its  familiarity, 
viz.,  the  transformation  of  iron  into  other 
substances,  will,  however,  serve  to  put  the 
reader  into  the  position  of  an  inquirer  wishing 
to  know  what  happens  to  iron  under  the 
circumstances  specified.  Now,  the  first  im- 
pression produced  when  such  cases  are 


CHEMICAL   CHANGE  25 

thought  over  seriously  is  one  of  marvel  at 
the  thoroughness  of  the  change.  The  more 
extensive  one's  experience — the  larger  the 
number  and  variety  of  the  transformations 
studied — the  more  wonderful  does  the 
phenomenon  appear,  even  to  those  whose  daily 
occupation  has  familiarized  them  with  such 
manifestations  of  the  properties  of  matter. 
Then  follows  naturally  the  question  whether 
chemical  change  really  is,  as  it  may  appear 
to  be  at  first  sight,  the  actual  transmutation 
of  one  form  of  matter  into  another — is  iron 
scale  or  iron  rust  simply  transmuted  iron  ? 

This  last  question  may  appear  a  very  simple 
one  ;  but  it  took  more  than  a  century's  work 
to  answer  it,  and  it  was  not  until  the  answer 
was  given  in  general  terms  that  Chemistry 
began  to  emerge  from  the  empirical  and  to 
pass  into  the  scientific  stage.  It  is  impossible 
within  the  compass  of  the  present  work  to 
attempt  the  historical  treatment  of  the 
subject :  suffice  it  to  say  that  the  answer  to 
the  above  question  is  in  the  negative.  Iron 
rust  or  iron  scale  is  not  simply  transmuted 
iron,  but  iron  in  combination  with  something 
else.  That  something  else  is  supplied  by  the 
air  or  water  ;  and  the  proof  of  this  is  furnished 
by  the  fact  that  the  iron  gains  in  weight  when 
it  is  changed  into  rust  or  scale.  The  nature 


26  CHEMISTRY 

of  the  other  component  need  not  concern  us 
just  now  ;  it  is  the  general  principle  which  is 
of  prime  importance  :  that  which  at  first  sight 
might  appear  to  be  a  case  of  transmutation 
turns  out  to  be  a  case  of  transformation  due 
to  the  combination  of  the  iron  with  another 
form  of  matter.  Two  further  questions  thus 
suggest  themselves : — Is  transmutation  in 
the  strict  sense  of  the  term  known  or  con- 
ceivable ?  Is  all  chemical  change  the  result 
of  the  combination  between  different  kinds 
of  matter  ?  In  answer  to  the  first  question, 
it  may  be  said  that  the  theoretical  develop- 
ment of  modern  Physics  in  connection  with 
the  constitution  of  matter  has  made  the 
notion  of  transmutation  conceivable,  but  the 
decision  from  this  point  of  view  must  be  left 
to  the  physicists.  Whether  cases  of  trans- 
mutation are  actually  known  is  a  chemical 
question  which  is  now  under  consideration. 

The  answer  to  the  second  question  is  of 
more  immediate  importance  for  the  adequate 
conception  of  the  nature  of  chemical  change. 
It  has  already  been  stated  that  the  change 
in  the  case  of  iron  under  the  conditions 
described  is  the  result  of  combination — of  the 
addition  to  the  iron  of  some  other  substance. 
This  other  component  of  iron  rust  and  iron 
scale  is  supplied  by  the  air  or  water  with  which 


CHEMICAL   CHANGE  27 

the  iron  is  brought  into  contact,  as  is  proved 
by  the  fact  that  iron  may  be  heated  or  exposed 
for  any  length  of  time  in  a  closed  vessel  from 
which  air  and  water  are  absent  without 
being  transformed  into  scale  or  rust.  It  may 
at  once  be  stated  that  this  other  component 
is  a  form  of  matter  known  to  chemists  as 
oxygen,  a  substance  which  under  ordinary 
conditions  is  a  colourless  gas,  but  which 
condenses  to  a  liquid  at  an  extremely  low 
temperature.  The  transformation  wrought 
by  chemical  change  appears  in  a  more  striking 
light  when  it  is  considered  that  from  solid 
iron  and  gaseous  oxygen  there  arises  a  black, 
brittle  scale,  or  a  reddish,  earthy  rust,  both 
solid,  differing  from  each  other,  and  absolutely 
unlike  their  generators.  Moreover,  underlying 
the  fact  that  two  different  forms  of  matter 
can  when  combined  give  rise  to  two  distinct 
substances  is  another  great  chemical  truth,  the 
bearing  of  which  will  be  considered  subse- 
quently. The  would-be  inquirer  will  also  want 
to  know  in  what  form  the  oxygen  exists  in  air 
and  water  respectively,  since  it  has  been 
described  as  a  gas,  and,  although  air  is  a  gas, 
water  is  a  liquid  until  its  temperature  is  raised 
sufficiently  to  convert  it  into  vapour  or  steam. 
So  that  the  suspicion  arises  that  the  oxygen 
may  not  be  in  the  same  condition  in  air  as  it 


28  CHEMISTRY 

is  in  water ;  and  another  point  is  thus  raised 
for  future  consideration.  And  if  the  reader 
is  prompted  by  that  spirit  of  active  inquiry 
which  is  the  prime  instigator  of  all  scientific 
progress,  he  will  demand  a  more  direct  proof 
of  the  statement  that  iron  rust  and  iron  scale 
contain  oxygen,  because  the  mere  gain  in 
weight  proves  nothing  more  than  that  the 
iron  has  taken  up  something  else.  But  the 
proof  of  this  assertion  necessitates  a  broader 
grasp  of  facts,  and  must  be  postponed  until 
the  general  outlook  has  been  widened.  In  the 
meantime,  the  main  point — whether  all  chemi- 
cal change  is  the  result  of  combination 
between  different  kinds  of  matter — must  be 
dealt  with. 

In  order  to  prepare  the  way  for  further 
developments,  it  may  at  once  be  stated  that 
the  answer  to  this  last  question  is  in  the 
negative.  The  reverse  process,  i.e.,  the  un- 
doing of  the  combination  between  different 
kinds  of  matter,  may  also  lead  to  trans- 
formations which  are  as  distinctly  chemical 
as  those  resulting  from  combination.  And 
when  we  come  to  inquire  more  deeply  into 
the  process  of  combination,  even  in  the  most 
familiar  and  apparently  straightforward  cases, 
such  as  the  rusting  of  iron,  it  is  found  that 
the  conditions  are  in  reality  very  complex — 


CHEMICAL    CHANGE  29 

so  complex  that  the  most  delicate  methods 
have  recently  had  to  be  employed  by  expert 
experimenters  in  order  to  determine  the  nature 
of  the  transformation,  and  whether  that  which 
at  first  sight  appears  to  be  a  simple  case 
of  direct  combination  between  iron  and 
oxygen  may  not  be  a  case  of  indirect  com- 
bination. In  fact,  the  formation  of  new 
substances  by  direct  combination  as  known 
to  chemists  is  generally  an  artificial  process, 
i.e.,  the  result  of  experimental  conditions 
imposed  by  the  experimenter.  Indeed,  the 
very  forms  of  matter  which  have  been  referred 
to  for  the  purposes  of  illustration  are,  humanly 
speaking,  artificial  products,  since  iron, 
although  found  to  a  limited  extent,  as  such, 
in  nature,  is  for  all  practical  purposes  obtained 
by  chemical  processes  from  its  naturally 
occurring  compounds ;  and  the  fact  that  the 
substance  of  iron  scale  is  found  in  large 
quantities  as  a  natural  mineral,  magnetite, 
suggests  interesting  lines  of  inquiry  respecting 
the  past  conditions  of  the  earth,  under  which 
the  iron  and  oxygen  were  enabled  to  combine 
so  as  to  produce  in  some  cases  magnetite, 
and  in  other  cases  the  substance  of  iron  rust, 
which  is  also  found  as  the  mineral  haematite, 
or,  in  combination  with  water  (hydrated), 
as  limonite.  Was  magnetite  formed  by  the 


30  CHEMISTRY 

direct  combination  of  iron  with  oxygen,  by 
the  action  of  water  upon  hot  iron,  or  by  some 
other  process  ?  Was  haematite  formed  from 
iron  by  some  process  analogous  to  that  of 
rusting  ?  These  questions  are  not  raised  for 
the  purpose  of  answering  them,  because  in  the 
present  state  of  knowledge  no  decisive  answer 
can  be  given,  but  in  order  to  illustrate  still 
further  the  all-embracing  scope  of  chemistry. 
From  the  simple  observation  that  iron  forms 
certain  compounds  with  oxygen,  there  arise 
questions  which  bring  Chemistry  into  the 
domain  of  Geology.  There  is,  in  fact,  a  science 
of  Geo-chemistry  yet  awaiting  development. 
Although  chemical  change  by  direct  com- 
bination is  now  a  rare  phenomenon  in  nature, 
it  may  have  been  and,  no  doubt,  was  the 
predominant  mode  of  material  transformation 
during  that  period  of  the  earth's  history  when 
our  globe  was  cooling  down  from  an  igneous 
condition.  But  high  temperature  chemistry 
at  the  present  time  is,  excepting  under 
volcanic  conditions,  an  artificial  chemistry, 
i.e.,  brought  about  by  human  artifice ;  and 
the  study  of  chemical  change  in  all  its  phases 
enables  us  to  state  that  direct  combination 
is  but  one  out  of  many  possible  modes  of 
bringing  about  such  transformations  of  matter. 
The  further  consideration  of  these  various 


AN   EXPERIMENTAL   SCIENCE      31 

modes  may  for  the  present  be  deferred ; 
but  it  must  at  once  be  realized  that  there  are 
in  operation  in  nature  processes  of  chemical 
change  other  than  by  direct  combination, 
which  are  everywhere  going  on,  subtly, 
silently,  and  generally  imperceptibly — light, 
air,  and  water  on  the  surface  of  the  earth, 
and  heat  and  pressure  below  are  slowly 
effecting  such  transformations  of  matter ; 
every  living  organism  is  an  active  centre  of 
chemical  change.  In  other  words,  the  con- 
sideration of  the  nature  of  chemical  change 
must  not  be  allowed  to  lead  to  the  belief  that 
the  process  is  purely  artificial.  There  is  a 
natural  chemistry  both  of  non-living  and  of 
living  matter  of  which  our  knowledge  is  still 
very  imperfect,  and  towards  the  better 
understanding  of  which  our  laboratory  studies 
are  gradually  leading  us. 

Chemistry  an  Experimental  Science. — A  very 
erroneous  impression  would  be  gained  if  it 
were  imagined  that  the  data  of  chemical 
science  can  be  obtained  by  direct  observation 
as  in  the  case  of  Astronomy,  or  of  the  biological 
sciences,  in  which  large  bodies  of  ready-made 
facts  are  offered  for  investigation.  Natural 
chemistry,  as  already  indicated,  is  both 
complicated  and  recondite.  The  mineral 
components  of  our  globe  represent  the  products 


32  CHEMISTRY 

of  chemical  changes  which  may  have  taken 
ages  for  their  completion ;  the  chemical 
processes  which  go  on  in  the  living  organism 
are  still  shrouded  in  mystery.  We  can 
produce  artificially  in  our  laboratories  large 
numbers  of  these  products  of  Nature's  labora- 
tory, mineral,  animal,  and  vegetable ;  but, 
great  as  are  the  achievements  of  modern 
Chemistry  in  this  direction,  it  must  not  be 
concluded  that  we  have  thereby  disclosed 
Nature's  methods.  We  cannot  compete  with 
Nature  in  her  scale  of  working — either  in  time, 
in  mass  of  material,  in  temperature,  or  in 
pressure.  We  do  not,  therefore,  go  to  Nature 
in  the  first  place  for  ready-made  facts  in  our 
endeavour  to  penetrate  the  inner  secrets  of 
those  properties  of  matter  upon  which  depend 
its  capabilities  of  undergoing  chemical  change  ; 
but  we  impose  our  own  conditions — in  other 
words,  we  cross-examine  Nature  by  experi- 
ment. By  experiment,  we  mean  trial — • 
the  observation  of  facts  obtained  under 
conditions  which  are  under  control,  and  which 
can,  therefore,  be  varied  in  known  ways. 
The  principle  involved  in  this  method  is  that 
which  is  responsible  for  the  development  of 
all  those  branches  of  science  which  are  not 
dependent  upon  the  direct  observation  of 
uncontrollable  phenomena.  By  studying 


AN    EXPERIMENTAL    SCIENCE      33 

simple  cases  under  controllable  conditions, 
and  observing  the  effects  of  changed  con- 
ditions, we  endeavour  to  connect  phenomena 
such  as  chemical  change  with  their  antecedent 
phenomena,  i.e.,  to  connect  effect  with  cause  ; 
and  so,  from  the  simpler  and  known  relation- 
ships established  by  the  experimental  method, 
we  pass  to  the  more  complex  and  unknown 
relationships  which  exist  under  natural  con- 
ditions. 

In  this  way  there  has  been  acquired  such 
knowledge  of  the  inherent  properties  of 
matter  as  could  never  have  been  acquired  by 
the  direct  observation  of  ready-made  facts. 
Chemistry — the  most  typical  of  the  experi- 
mental sciences — has  so  far  penetrated  the 
inner  mysteries  of  matter  as  to  have  called 
into  existence  an  infinitude  of  new  compounds, 
i.e.,  of  forms  of  matter  absolutely  unknown 
in  nature.  In  claiming  the  achievements  of 
synthetical  chemistry  as  triumphs  of  modern 
science,  it  must,  of  course,  be  understood 
that  no  claim  is  made  to  our  having  mastered 
Nature  in  the  sense  of  conquering  matter. 
Our  chemistry  is  not  an  unnatural  chemistry  ; 
all  that  has  been  achieved  has  been  made 
possible  only  by  our  having  learnt  those 
potentialities  of  matter  which,  in  their  ultimate 
essence,  may  for  ever  elude  our  methods  of 


34  CHEMISTRY 

investigation.  Every  artificial  substance  pro- 
duced in  our  laboratories  or  factories  is  but 
the  materialization  of  potentialities  already 
inherent,  and  which,  so  far  as  we  know,  may 
have  been  present  in  matter  from  the  begin- 
ning of  the  existing  order  of  things  : — "  Yet 
Nature  is  made  better  by  no  mean,  but  Nature 
makes  that  mean  :  so,  o'er  that  art,  which, 
you  say,  adds  to  Nature,  is  an  art  that  Nature 
makes." 

It  must  be  clearly  understood,  therefore, 
that  Chemistry  as  a  science  cannot  be  learnt, 
in  the  strict  sense  of  the  word,  by  merely 
reading  about  it.  The  would-be  student  may 
be  informed  of  the  existing  state  of  knowledge 
through  books  of  a  descriptive  or  historical 
character ;  but  he  must  realize  that  all  the 
information  given — all  the  general  principles, 
all  the  laws  and  generalizations  that  we  are 
enabled  to  enunciate,  have  been  based  on  facts 
gleaned  by  laboratory  work.  That  is  why  the 
foundations  of  modern  Chemistry  are  said  to 
be  well  and  truly  laid.  The  apparently  simple 
observation  from  which  we  set  out — the 
rusting  of  iron — is  in  reality  an  experiment, 
because  the  transformation  is  under  control. 
For  instance,  we  can  modify  the  state  of 
aggregation  of  the  iron  by  reducing  it  to  fine 
particles  by  a  file,  or,  if  brittle,  we  can  grind 


AN   EXPERIMENTAL    SCIENCE      35 

it  to  a  fine  powder  (iron  swarf) ;  and  if  some  of 
this  finely  divided  iron  is  exposed  for  some 
days  in  a  confined  volume  of  air,  such  as  is 
contained  in  a  bottle  inverted  over  water,  we 
can  find  out  that  the  air  as  a  whole  is  not 
absorbed,  but  only  a  certain  proportion  of  it, 
about  one-fifth  of  its  volume.  So  that  the 
oxygen  which  combines  with  the  iron  is  picked 
out  by  the  latter  by  a  selective  process,  and 
anything  that  is  not  oxygen  is  left  uncom- 
bined,  and  the  rusting  ceases  as  soon  as  all 
the  oxygen  is  used  up.  An  experiment  of 
this  kind  teaches  us,  therefore,  that  chemical 
change  is  preferential,  inasmuch  as  the  iron 
combines  with  only  one  form  of  matter  present 
in  the  air,  and,  under  the  conditions  of  the 
experiment,  refuses  to  combine  with  any  other 
substance  contained  in  the  bottle.  From  this 
it  further  follows  that  air  contains  some- 
thing besides  oxygen  ;  and  the  question  as 
to  the  condition  in  which  oxygen  is  present 
in  the  air,  which  was  previously  raised  (p.  28) 
is  thus  answered  to  the  extent  that  it  is 
permissible  to  state  that  it  is  in  a  form  easily 
removed  by  iron  at  ordinary  temperatures. 
The  bearing  of  this  will  be  seen  subsequently. 
It  must  be  noted,  also,  that  it  has  been 
throughout  assumed  as  a  condition  of  rusting 
that  the  air  must  be  moist,  i.e.,  that  it  con- 


36  CHEMISTRY 

tains  the  vapour  of  water.  That  statement  is 
based  upon  experiment,  because  if  we  remove 
all  the  water  vapour  from  the  air  by  absorbent 
substances,  of  which  many  are  known  to 
chemists,  then  no  rusting  takes  place,  although 
there  may  be  oxygen  present.  So  that 
oxygen  and  water  vapour  are  both  essential 
for  the  production  of  the  chemical  change  in 
this  case.  Moreover,  there  is  also  present 
in  air  a  small  quantity  of  another  gaseous 
substance,  which  is  known  as  carbon  dioxide 
— the  gas  which  is  familiar  to  all  as  giving 
rise  to  the  evolution  of  bubbles,  i.e.,  the  effer- 
vescence when  a  bottle  of  mineral  water 
or  "  sparkling "  wine  is  opened.  This  gas 
is  present  only  to  the  extent  of  three  or  four 
volumes  in  ten  thousand  volumes  of  air,  so 
that  refined  and  delicate  methods  of  measur- 
ing its  quantity  have  to  be  adopted.  It  is 
believed  by  some  experimenters  that  the 
co-operation  of  this  gas  with  oxygen  and  water 
vapour  is  also  essential  for  the  rusting  of  iron, 
while  others  have  come  to  the  conclusion  that 
the  iron  itself  can  exist  in  either  a  sensitive 
or  insensitive  state  with  respect  to  its  power 
of  combining  with  oxygen,  and  that  carbon 
dioxide  is  not  essential.  All  these  facts  are 
mentioned  in  further  illustration  of  the 
importance  of  the  experimental  method,  and 


AN    EXPERIMENTAL    SCIENCE      37 

to  enforce  the  lesson  already  inculcated,  that 
the  apparent  simplicity  of  a  familiar  chemical 
change  disappears  when  Nature  is  rigidly  cross- 
examined  under  controllable  conditions  with 
the  object  of  finding  out  how  and  why  such 
transformation  takes  place  in  any  particular 
case. 

It  must  not  be  imagined  that  the  resources 
of  experiment  are  exhausted  when,  as  in  the 
foregoing  illustration,  the  chemical  trans- 
formation of  iron  has  been  traced  to  its 
faculty  of  combining  with  oxygen.  In  addi- 
tion to  the  facts,  a  principle  has  been  revealed 
— the  principle  that  iron,  regarded  as  a  form 
of  matter,  has  its  likes  and  dislikes,  since  it 
selects  the  oxygen  from  the  air,  and  leaves 
four-fifths  of  some  uncombined  gas  behind. 
That  gas,  it  may  at  once  be  parenthetically 
noted  for  future  reference,  consists  mainly  of 
another  form  of  matter  known  as  nitrogen. 
As  a  deduction  from  the  facts  already 
observed,  the  question  next  arises  whether 
iron  shows  preference  for  other  substances. 
Thus  we  might  proceed  to  test  this  question 
experimentally  by  mixing  iron  with  other 
familiar  substances,  such  as  sand,  or  sugar,  or 
charcoal,  or  chalk,  or  sulphur.  To  make  the 
question  more  searching  we  should  naturally 
in  such  a  case  give  the  iron  every  chance,  by 


38  CHEMISTRY 

using  the  finely-divided  substance  in  the  form 
of  filings  or  dust,  and  we  should  grind  this  up 
with  the  other  materials.  Under  these  cir- 
cumstances, we  should  find  that  nothing  more 
than  mixtures  would  be  obtained — the  pro- 
ducts would  not  come  within  the  conception 
of  chemical  change,  because  the  iron  is  still 
present  as  such,  and  could  be  separated  from 
the  sand  or  sugar,  etc.,  by  means  of  a  magnet ; 
or  the  sugar  could  be  dissolved  out  by  water, 
leaving  the  iron  behind ;  and  so  with  the 
other  mixtures,  separation  could  be  effected 
by  appropriate  methods. 

But  if  iron  is  ground  up  with  that  metallic 
looking  solid  known  as  iodine — a  substance 
obtained  from  the  ashes  of  sea-weeds,  and 
familiar  in  pharmacy — we  find  that,  if  a  suffi- 
cient quantity  of  iodine  is  used,  a  grey  product 
is  obtained  from  which  iron  cannot  be  readily 
separated ;  the  iron  and  the  iodine  have  lost 
their  individuality  through  chemical  change. 
Thus,  under  the  conditions  in  which  iron 
refuses  to  combine  with  sugar  or  charcoal 
or  the  other  substances,  it  combines  with 
iodine,  and  the  principle  of  preferential  action 
is  thus  extended.  This  is  what  in  old  time 
chemistry  would  be  expressed  by  saying  that 
the  iron  had  an  "  affinity  "  for  oxygen  and 
iodine,  but  not  for  nitrogen,  or  sand,  or  chalk, 


AN    EXPERIMENTAL    SCIENCE     39 

or  any  of  the  other  substances.  And  now, 
prosecuting  the  inquiry  still  further,  we 
should  find  that  the  application  of  heat  may 
bring  about  combination  in  the  case  of  the 
sulphur-iron  mixture,  but  in  no  other  of  the 
mixtures  referred  to,  although  we  should 
discover  incidentally  that  the  sugar  was 
totally  transformed  on  heating,  with  the 
production  of  charcoal,  that  the  sand  and 
charcoal  were  unchanged,  and  that  the  chalk, 
although  apparently  unaltered  in  appearance, 
had,  if  strongly  heated,  become  transformed 
into  another  substance,  lime.  When  heated, 
the  mixture  of  iron  and  sulphur  suddenly 
begins  to  glow — developing  heat  by  the 
combination  of  the  materials — and  the  pro- 
duct, when  cold,  consists  of  a  dark  coloured, 
brittle  solid  from  which,  provided  sufficient 
sulphur  has  been  added,  iron  cannot  be  readily 
separated.  Here  again,  chemical  change  has 
taken  place. 

It  will  be  obvious  from  these  illustrations 
that  Chemistry  is  an  art  as  well  as  a  science ; 
the  carrying  out  even  of  simple  experiments 
necessitates  manual  dexterity,  skill,  judgment, 
and  a  knowledge  of  the  various  contrivances 
or  forms  of  apparatus  available  for  particular 
purposes.  And  so  we  are  brought  back  to  the 
fundamental  proposition  that  our  scientific 


40  CHEMISTRY 

knowledge  has  been  built  on  a  foundation 
furnished  by  the  art  of  the  experimentalist. 
We  can  learn  from  books  what  are  the  general 
scientific  conclusions  at  any  particular  period  ; 
but  no  amount  of  reading  will  make  a  learner 
into  a  proficient  chemist. 


CHAPTER   II 

CHEMICAL      COMBINATION      AND      MECHANICAL 

MIXTURE AIR    A    MIXTURE     AND     NOT    A 

COMPOUND — PHYSICAL       SEPARATION      OF 
THE   COMPONENTS    OF   AIR 

Chemical  Combination  and  Mechanical  Mix- 
ture.— The  consideration  of  those  profound 
modifications  in  matter  which  result  from 
chemical  change  will  have  made  it  evident 
that  a  clear  distinction  must  be  made  between 
the  products  of  chemical  combination  and 
mechanical  mixtures.  This  point  is  very 
generally  misunderstood  by  the  uninstructed, 
and  is  often  a  stumbling-block  to  the  student 
on  his  first  introduction  to  chemical  science. 
The  difficulty  arises  from  the  circumstance 
that  the  product  arising  from  a  combination 
of  two  different  substances  does  not,  as 
common  sense  might  lead  us  to  suppose, 
partake  of  the  characters  of  both  components, 
but  is  a  totally  distinct  form  of  matter.  The 
common  sense  notion  applies  to  mechanical 
mixtures,  for  these  do  partake  of  the  characters 

41 


42  CHEMISTRY 

of  their  components — their  properties  are 
intermediate  between  those  of  the  substances 
mixed,  and  can  be  made  to  vary  indefinitely 
by  varying  the  proportions  of  the  ingredients. 
But  these  ingredients,  whatever  they  may  be, 
are  always  present  as  such  in  the  mixture. 
However  finely  we  may  grind  up  a  mixture  of, 
let  us  say,  iron  dust  and  chalk,  we  can  always 
pick  out  the  iron  by  a  magnet,  because  it  is 
a  property  of  iron  to  be  attracted  by  a  magnet, 
while  chalk  is  not  thus  attracted.  Moreover, 
a  mechanical  mixture  is  never  homogeneous  : 
such  mixtures  may  be  made  apparently 
homogeneous  by  grinding  to  a  very  fine 
powder,  but,  if  we  examine  some  of  this  powder 
under  a  microscope,  the  different  particles 
of  the  components  can  easily  be  distinguished. 
The  product  of  chemical  combination,  on  the 
other  hand,  is  always  homogeneous — its 
particles,  however  finely  divided  by  mechani- 
cal means,  are  all  alike  in  appearance  and 
properties.  The  mixture  of  iron  dust  and 
sulphur  referred  to  in  the  last  chapter  is 
visibly  a  mixture  when  sufficiently  magnified  ; 
but  if,  after  heating,  the  product  of  com- 
bination is  ground  up  to  a  powder,  no 
magnification  will  reveal  either  iron  or  sulphur 
provided  the  materials  were  present  in  the  right 
proportions. 


COMBINATION    AND    MIXTURE    43 

This  last  condition  introduces  a  new  set  of 
considerations — the  quantitative  relations  be- 
tween the  combining  materials.  Herein  we 
shall  find  a  characteristic  of  chemical  com- 
bination of  such  fundamental  importance 
that  this  part  of  the  subject  must  be  fully 
dealt  with  at  a  later  stage,  when  the  general 
nature  of  chemical  change  has  been  more 
thoroughly  grasped.  In  the  meantime,  it 
will  be  instructive  to  give  one  or  two  illus- 
trations of  the  popular  misconception  of 
chemical  transformation.  We  often  see  it 
stated,  for  example,  that  metals  are  extracted 
from  their  ores,  or  that  dyes,  perfumes,  ex- 
plosives, etc.,  are  extracted  from  coal-tar. 
To  the  uninitiated,  these  statements  might 
convey  the  impression  that  the  metal,  say 
iron,  is  present  as  such  in  the  ore,  or  that  the 
dyes,  etc.,  are  contained  in  the  tar,  and  that 
the  process  of  extraction  is  nothing  more 
than  a  sort  of  mechanical  separation  of  the 
metal  or  the  dyes  from  the  other  materials 
with  which  they  are  mixed.  Nothing  could 
be  further  from  the  truth  than  this  conception 
of  the  process.  There  is  no  free  iron  in  iron 
ore ;  there  are  no  ready  formed  dyes  or 
perfumes  in  tar.  The  liberation  of  iron  from 
the  oxygen  with  which  it  is  chemically  com- 
bined in,  let  us  say,  haematite  is  the  result  of 


44  CHEMISTRY 

chemical  transformation  which  the  ore  is 
made  to  undergo  by  heating  it  with  some 
other  substance,  such  as  the  carbonaceous 
fuel  referred  to  in  the  last  chapter,  which 
enters  into  chemical  combination  with  the 
oxygen.  That  is  why  iron  was  previously 
declared  to  be  a  product  of  artificial  chemistry 
— in  this  case  we  call  it  a  metallurgical 
process,  because  iron  belongs  to  a  class  of 
substances  known  as  metals.  So,  also,  are 
the  dye-stuffs,  etc.,  artificial  products  ;  they 
are  obtained  from  certain  substances  con- 
tained in  and  separated  from  the  tar,  but  the 
generating  substances — the  raw  materials — • 
are  not  themselves  dye-stuffs.  The  latter 
arise  from  their  non-tinctorial  generators  by  a 
series  of  operations  which  involve  chemical 
transformation  at  every  stage.  This  last 
case  is  among  the  most  striking  illustrations 
of  the  marvellous  transformations  brought 
about  by  chemical  change.  Even  when  a 
naturally  occurring  ore  does  contain  a  metallic 
substance  which,  like  gold,  has  to  be  separated 
from  the  other  materials  with  which  it  is 
mixed,  the  extraction  is  very  seldom  a  purely 
mechanical  process  :  chemical  extraction  is 
generally  resorted  to. 

Although  the  fundamental  distinction  be- 
tween chemical  combination  and  mechanical 


COMBINATION    AND    MIXTURE    45 

mixture  has,  no  doubt,  been  made  clear  by 
the  illustrations  given,  it  is  not  always  easy 
to  decide  off-hand  whether  any  given  substance 
is  a  mixture  or  a  true  chemical  compound. 
Let  it,  in  the  first  place,  be  realized  that  since 
Chemistry  has  for  its  scope  the  study  of  the 
transformations  of  matter  in  all  its  forms, 
the  particular  state  of  physical  aggregation  of 
the  matter  concerned  in  any  case  of  chemical 
change  is  of  minor  importance  from  the 
chemical  point  of  view.  From  every  day 
experience  the  reader  has  been  made  familiar 
with  the  existence  of  matter  in  the  three 
forms  known  respectively  as  solid,  liquid, 
and  vapour,  or  gas.  The  interconvertibility 
of  these  forms  is  generally  dealt  with  from  the 
physical  side  in  works  on  Physics,  although, 
as  has  already  been  pointed  out,  the  specific 
properties  of  different  forms  of  matter  with 
respect  to  the  conditions  which  determine 
their  state  of  aggregation  are  also  of  chemical 
significance.  Illustrations  of  chemical  change 
resulting  from  combination  between  different 
kinds  of  matter  in  different  states  of  aggrega- 
tion have  already  been  given.  Thus,  solid  iron 
combines  with  gaseous  oxygen  and  with  solid 
iodine  giving  rise  to  chemical  compounds. 
The  selection  of  the  oxygen  from  the  air  by 
iron  wucn  it  rusts  has  been  referred  to  as  a 


46  CHEMISTRY 

fact  indicating  that  the  oxygen  is  present  in 
the  air  in  an  easily  removable  form.  But 
this  fact  does  not  in  itself  decide  the  question 
whether  the  oxygen  in  the  air  is  chemically 
combined  with  the  nitrogen  or  the  other 
substances  which  are  known  to  be  present  in 
air.  It  does  not  answer  the  question — Is  air 
a  chemical  compound,  or  a  mechanical  mix- 
ture ?  We  know  of  many  true  chemical 
compounds  which  are  gaseous  at  ordinary 
temperatures,  and  we  know  also  that  iron 
and  other  substances  can  decompose  some  of 
these  compounds,  i.e.,  can  undo  the  chemical 
combination  by  taking  out  one  of  the  com- 
ponents and  leaving  the  other.  This  will  be 
made  clearer  as  we  proceed ;  but  the  bare 
statement  of  these  facts  will  suffice  at  present 
to  enforce  the  lesson  that  the  scientific  inter- 
pretation of  experimental  evidence  requires 
both  caution  and  judgment. 

Air  a  Mixture,  and  not  a  Compound. — It  is, 
of  course,  well  known  that  air  is  not  a  chemical 
compound,  but  a  mixture  of  oxygen  and 
nitrogen  with  small  quantities  of  other  gaseous 
substances,  such  as  carbon  dioxide,  water 
vapour,  and  traces  of  a  few  other  gases,  which 
will  be  spoken  of  in  a  later  chapter.  If, 
therefore,  the  observation  that  rusting  iron 
withdraws  the  oxygen  is  not  decisive,  how, 


THE    COMPONENTS    OF   AIR       47 

then,  it  may  be  asked,  has  this  conclusion 
been  arrived  at  ?  So  far  as  mere  homo- 
geneity is  concerned,  the  air  answers  to  the 
definition  of  a  chemical  compound  :  it  has 
the  same  properties  from  whatever  part  of 
the  world  it  comes.  The  rusting  iron  experi- 
ment gives  within  quite  narrow  limits  the 
same  quantitative  result  with  air  from  any 
quarter  of  the  globe,  if  the  volume  of  oxygen 
withdrawn  is  measured.  The  conclusion  that 
air  is  a  mixture  is  based  upon  convergent  lines 
of  evidence,  the  consideration  of  every  one 
of  which  will  introduce  us  to  new  general 
principles.  Two  or  three  of  these  lines  may 
be  advantageously  followed  up  now. 

In  the  first  place,  let  us  ask  whether  there 
is  any  criterion  of  chemical  combination 
beyond  that  general  change  in  character  of  the 
transformed  substance  which  has  so  far  been 
considered.  To  put  the  case  in  another  way 
— is  there  any  method  of  ascertaining,  apart 
from  the  appearance  and  properties  of  the  re- 
sulting product,  whether  two  substances  when 
mixed  together  enter  into  chemical  combina- 
tion, or  remain  simply  in  admixture  ?  There 
is  one  such  indication  of  chemical  combination 
which,  when  observed,  is  a  very  sure  sign 
that  something  more  than  mechanical  mixture 
has  taken  place,  and  that  is  the  development 


48  CHEMISTRY 

of  heat.  This  is  a  point  of  fundamental 
significance,  and  will  require  further  elabora- 
tion. It  is  not  always  possible  to  detect  the 
heat  of  chemical  combination,  because  in 
many  cases  the  transformation  takes  place 
very  slowly,  and  the  heat  is  dissipated  too 
rapidly  to  enable  us  to  measure  it.  The  com- 
bination of  iron  with  oxygen  is  a  case  in  point. 
There  is  no  doubt  that  heat  is  developed  when 
iron  rusts,  but  that  is  a  case  of  slow  chemical 
combination.  Now,  although  the  state  of 
physical  aggregation  of  the  matter  entering 
into  combination  is,  as  already  stated,  of 
minor  importance  so  far  as  concerns  the 
nature  of  the  final  product,  yet  it  is  obvious 
that  the  state  of  physical  aggregation  may 
influence  the  rate  of  combination.  In  the 
case  of  a  solid  combining  with  another  solid, 
or  with  a  liquid  or  a  gas,  it  is  evident  that  the 
more  intimately  the  combining  materials  are 
brought  into  contact  the  more  rapid  will  be 
the  chemical  combination.  That  is  why,  in 
such  experiments  with  iron  as  have  been 
described,  it  was  thought  desirable  to  use  the 
metal  in  a  finely  divided  form.  It  is  simply 
a  case  of  presenting  as  large  a  surface  as 
possible  to  the  other  substance.  But,  how- 
ever finely  we  may  divide  iron  by  mechanical 
processes,  it  is  not  possible  to  increase  the  rate 


THE    COMPONENTS    OF   AIR       49 

of  its  combination  with  oxygen  to  a  sufficient 
extent  to  enable  us  to  detect  the  rise  of 
temperature  on  rusting.  All  that  can  be  said 
is  that  iron  sheet  would  rust  more  rapidly 
than  a  solid  block,  and  that  iron  filings  or 
iron  dust  would  rust  more  rapidly  than  either 
block  or  sheet  iron.  It  is  possible — although 
not  easy  practically — to  prove  this  experi- 
mentally by  measuring  the  rate  of  absorp- 
tion of  oxygen  from  air  by  iron  in  different 
states  of  division.  It  is  possible,  also,  by  a 
chemical  process  to  obtain  iron  in  such  an  ex- 
tremely fine  state  of  division  that  it  possesses 
what  are  known  as  "  pyrophoric  "  properties, 
becoming  red  hot  on  being  shaken  into  the 
air  from  the  tube  which  contains  it.  In  this 
case,  the  combination  between  the  micros- 
copic particles  of  iron  and  the  atmospheric 
oxygen  takes  place  so  rapidly  that  the  evolu- 
tion of  heat  produces  a  visible  effect. 

The  development  of  heat  as  a  criterion  of 
chemical  combination  is  thus  likely  to  be 
most  observed  in  cases  where  the  combining 
materials  are  brought  into  the  most  intimate 
contact.  No  state  of  physical  aggregation 
can  insure  more  intimate  contact  than  the 
gaseous  state,  for  this  is  the  most  mobile 
condition  of  matter,  and  gases  mix  freely 
with  each  other  in  all  proportions.  Now,  the 


50  CHEMISTRY 

main  constituents  of  the  air,  oxygen  and 
nitrogen,  are  gases  ;  they  can  be  obtained  in 
various  ways  by  chemical  processes,  and  they 
can  be  mixed  together  in  the  same  proportions 
as  those  in  which  they  exist  in  the  air,  viz., 
four  volumes  of  nitrogen  and  one  of  oxygen. 
The  resulting  product  is  a  gas  having  all  the 
properties  of  air ;  no  change  of  temperature 
takes  place,  and  the  characters  of  the  mixture 
are  intermediate  between  those  of  its  compon- 
ents. That  is  one  reason  why  air  is  regarded 
as  a  mixture,  and  not  as  a  compound. 

The  homogeneity  of  product  in  this  case, 
therefore,  is  in  a  sense  accidental — it  is  the 
result  of  the  extreme  mobility  of  the  gaseous 
form  of  matter.  It  may  be  asked,  now,  if  the 
homogeneity  of  air — which  is  supposed  to 
be  characteristic  of  a  true  chemical  com- 
pound— is  thus  a  violation  of  that  criterion 
of  chemical  union  which  has  been  previously 
set  up.  Is  the  homogeneity  real,  or  is  it 
only  apparent  ?  This  question  may,  in  the 
first  place,  be  answered  hypothetically. 
Supposing,  by  some  magnification  of  the 
power  of  vision,  it  were  possible  to  see  the 
actual  particles  of  which  a  gas  is  made  up, 
different  gaseous  forms  of  matter  might  be 
expected  to  present  different  appearances. 
The  particles  might  be  quite  dissimilar  to  an 


THE   COMPONENTS   OF   AIR       51 

imaginary  being  with  such  an  exalted  sense 
of  vision — they  might  differ  in  size  or  shape  or 
weight,  or  they  might  be  moving  about  with 
different  velocities.  Thus,  our  hypothetical 
being  might  be  supposed  to  know  an  oxygen 
particle  from  a  nitrogen  particle ;  and  if  a 
sample  of  air  were  submitted  to  him  for 
examination,  what  would  he  find  ?  He  would 
see  the  oxygen  particles  and  the  nitrogen 
particles  mixed  up,  moving  about  among 
each  other,  and  bombarding  the  sides  of  the 
containing  vessel,  colliding  and  rebounding — 
all  in  a  higgledy-piggledy  way ;  but  the  two 
kinds  of  particles  would  throughout  their 
evolutions  remain  distinct,  each  after  its 
kind  ;  there  would  be  no  fusion  together  or 
combination,  and  each  kind,  even  if  tem- 
porarily deformed  by  collisions,  would  preserve 
its  weight  as  well  as  its  average  rate  of 
movement. 

This  ideal  picture  of  the  inner  state  of  affairs 
in  gaseous  matter  is  a  physical  conception,  and 
is  in  harmony  with  all  those  general  properties 
of  gases  which  the  student  learns  from  the 
science  of  Physics.  If  the  picture  be  a  true 
representation  of  the  facts,  it  follows  that  the 
homogeneity  of  air  is  apparent,  and  not  real ; 
its  particles,  could  we  see  them,  would  not 
be  all  alike  as  they  would  be  if  it  were  a 


52  CHEMISTRY 

true  compound.  The  only  approach  towards 
homogeneity  that  could  be  realized  by  such 
a  mixture  would  be  the  possession  by  equal 
volumes  taken  at  random  of  absolutely  the 
same  number  of  particles  of  the  two  gases. 
It  is  for  the  sake  of  simplifying  the  argument 
that  attention  has  been  concentrated  upon 
these  two  components,  because  the  oxygen  and 
nitrogen  together  make  up  the  main  bulk  of 
the  air.  But  what  has  been  said  with  respect 
to  these  two  gases  is  true  for  the  other  minor 
components,  such  as  water  vapour  and  carbon 
dioxide,  and  all  the  other  gases  which  exist 
in  mere  traces.  Our  imaginary  being  with 
supernatural  power  of  vision  would  be  likewise 
capable  of  distinguishing  between  and  follow- 
ing the  migrations  of  the  fewer  particles  of 
water  vapour  and  carbon  dioxide  in  the  course 
of  their  wanderings  among  the  greater  crowd 
of  oxygen  and  the  still  greater  crowd  of 
nitrogen  particles.  With  these  other  com- 
ponents, there  is  no  combination  in  the 
chemical  sense. 

So  much  for  the  hypothetical  answer  to  the 
question  whether  air  is  really  homogeneous. 
Of  course,  the  ideal  supernatural  vision  is 
unattainable  by  any  human  contrivance,  so 
that  direct  proof  of  heterogeneity  is  not  to 
be  looked  for  by  any  such  means.  But  if 


THE    COMPONENTS    OF   AIR       53 

heterogeneity  can  be  proved  experimentally, 
then  it  will  be  admitted  that  there  is  justifica- 
tion for  the  hypothetical  answer.  There  are 
many  such  proofs — that  which  is  unrealizable 
visually,  viz.,  the  discrimination  between  the 
different  kinds  of  particles,  can  be  realized 
by  other  means  ;  and  it  is  of  the  utmost 
importance  to  note  that  the  means  about  to 
be  indicated  are  non-chemical.  The  import- 
ance of  this  reservation  will  become  apparent 
when  it  is  restated  that  any  attempt  to 
separate  the  components  by  the  action  of  some 
other  form  of  matter  which  exerts  a  selective 
action — as  in  the  case  of  rusting  iron — is 
always  open  to  the  suspicion  that  there  may 
have  been  chemical  decomposition.  As  al- 
ready pointed  out,  the  removal  of  the  oxygen 
or  nitrogen  or  of  any  other  constituent  gas  by 
chemically  combining  it  with  some  other 
substance  leaves  the  question  of  the  original 
condition  of  the  oxygen,  etc.,  in  the  air  an 
open  one. 

A  simple  observation  will  serve  to  show  that 
one  of  the  components,  viz.,  the  water 
vapour,  is  not  chemically  combined.  It  is,  no 
doubt,  a  familiar  fact  with  those  who  wear 
spectacles  or  eye-glasses  that,  on  coming 
from  the  cold  outer  air  suddenly  into  a  warm 
room,  the  glasses  become  dimmed  by  a  deposit 


54  CHEMISTRY 

of  moisture — a  deposition  of  dew  upon  the 
glass.  The  same  thing  is  observed  if  we  take 
a  glass  of  iced  water  into  a  warm  room  ;  the 
outside  of  the  glass  becomes  covered  with 
moisture.  That  means  that  our  supernatural 
being  was  right  when  he  observed  particles  of 
water  vapour  moving  about  among  the  oxygen 
and  nitrogen  particles.  The  interpretation  of 
the  observation  is  that  the  water  vapour  is 
only  retained  in  the  air  because  the  air  is 
warm  ;  warm  air  contains  more  water  than 
cold  air,  so  that,  when  warm  air  is  cooled  by 
contact  with  cold  glass,  it  deposits  some  of  its 
water  vapour  on  the  glass  in  the  form  of 
droplets  of  liquid  water  or  dew.  The  separa- 
tion of  water  as  such  from  the  air  by  the  mere 
lowering  of  temperature  indicates  that  the 
water  was  there,  although  in  an  invisible 
or  vaporous  state :  had  the  water  been 
chemically  combined  with  any  other  con- 
stituent of  the  air  it  could  not  possibly  have 
been  liberated  by  the  simple  process  of  cooling. 
The  proof  that  the  nitrogen  and  oxygen 
in  air  are  not  chemically  combined  involves 
the  application  of  methods  which  are  probably 
unfamiliar  to  the  general  reader.  Their  very 
unfamiliarity  makes  them  instructive,  because 
their  consideration  will  bring  us  into  contact 
with  other  fundamental  properties  of  matter. 


THE    COMPONENTS    OF   AIR       55 

Let  it  be  borne  in  mind  that,  in  this  particular 
case,  we  are  dealing  with  matter  in  the  gaseous 
form.  The  particles  of  matter  in  this  state 
possess,  as  we  have  already  explained,  perfect 
freedom  of  movement ;  and  our  imaginary 
being  has  been  supposed  to  see  the  different 
kinds  of  particles  moving  about  with  different 
velocities.  Now,  the  average  speed  of  these 
particles  is  a  property  which  is  dependent 
upon  the  nature  of  the  gaseous  substance  ; 
it  is,  in  fact,  a  physico-chemical  property, 
and  is  connected  with  the  relative  weights 
of  the  particles.  We  shall  have  to  consider 
later  how  these  weights  are  ascertained  ;  but 
it  is  easy  even  at  this  stage  to  form  a  mental 
picture  of  light  and  heavy  particles  all  mixed 
up  together,  the  lighter  particles  moving  more 
rapidly  than  the  heavier  particles.  If,  there- 
fore, the  oxygen  and  nitrogen  particles  are 
independent  entities,  and  not  chemically 
combined,  and  if  the  two  kinds  of  particles 
have  different  weights,  and  are  within  a  given 
volume  of  air  moving  with  different  velocities, 
then  a  process  of  mechanical  sorting  seems 
conceivable. 

The  foregoing  conception  can  be  verified 
experimentally,  because  it  happens  that  the 
nitrogen  particles  are  a  little  lighter  than  the 
oxygen  particles — in  fact,  a  nitrogen  particle 


56  CHEMISTRY 

has  seven-eighths  the  weight  of  a  comparable 
oxygen  particle.  If  we  could  pass  air  through 
a  sieve  with  very  small  meshes — small  enough 
to  bear  comparison  with  the  actual  size  of  the 
particles,  and  not  large  enough  to  allow  the 
whole  mixture  of  particles  to  pass  en  masse 
through  the  interstices — then  more  of  the  light 
than  of  the  heavy  particles  would  get  through 
in  a  given  time,  because  the  lighter  particles 
are  moving  the  more  rapidly.  The  air  which 
passed  through  such  a  sieve  ought,  conse- 
quently, to  be  richer  in  nitrogen,  and  the  air 
which  was  left  behind  ought  to  be  richer  in 
oxygen.  Now,  the  fine-meshed  sieve  which 
enables  this  conception  to  be  verified  may  be 
any  substance  with  extremely  minute  pores, 
such  as  unglazed  porcelain,  or  plaster  of 
Paris.  Of  course,  the  reader  is  familiar  with 
the  physical  fact  that  air  cannot  be  confined 
in  a  porous  vessel  if  it  is  under  pressure,  or  if 
there  is  no  air  outside  the  vessel  to  balance 
the  pressure  of  the  air  within.  If,  therefore, 
air  be  drawn  through  a  glass  tube  containing 
a  plug  or  diaphragm  of  some  fine-pored 
material,  the  proportions  of  the  gases  will  be 
altered — we  should  draw  out  of  the  tube  an 
air  containing  more  nitrogen  than  the  normal 
proportion,  and  there  would  be  left  behind  an 
air  containing  more  than  one-fifth  of  its  volume 


THE    COMPONENTS    OF    AIR       57 

of  oxygen.  Such  a  purely  mechanical  separa- 
tion as  this  is  another  proof  that  the  nitrogen 
and  oxygen  in  air  are  not  chemically  combined. 

It  will  be  noted  that,  throughout  this 
discussion  of  the  question  whether  air  is  a 
mixture  or  a  compound,  it  has  been  assumed 
that  the  gaseous  substances,  oxygen,  nitrogen, 
etc.,  consist  of  "  particles."  This  conception 
of  the  constitution  of  gases  has  been  referred 
to  as  a  physical  conception,  and  the  further 
development  of  the  idea  in  its  physical 
aspects  must  be  pursued  in  works  on  Physics. 
We  are  here  brought  face  to  face  with  one  of 
the  most  striking  examples  of  the  inter- 
dependence of  two  branches  of  science.  So 
far  as  the  conception  has  been  made  use  of 
in  this  chapter,  the  main  object  has  been  to 
introduce  the  reader  to  the  current  view  that 
the  substances,  nitrogen,  oxygen,  etc.,  although 
gaseous  under  ordinary  conditions,  are  not 
to  be  looked  upon  as  continuous,  but  as 
discontinuous  in  structure.  The  supposed 
magnification  up  to  the  stage  of  visibility  has 
been  imagined  to  reveal  a  discrete  or  granular 
constitution — the  gases  have  been  supposed 
to  consist  of  extremely  minute  particles. 
This  is  the  modern  view  of  the  constitution  of 
matter  in  all  states  of  aggregation. 

It  is,  historically  speaking,  a  very  ancient 


58  CHEMISTRY 

conception,  but  it  has  been  put  upon  a  scientific 
basis  in  modern  times  by  the  joint  labours  of 
chemists  and  physicists.  All  forms  of  matter, 
solid,  liquid  and  gaseous,  if  we  could  but  see 
into  their  inner  constitution,  would  be  found 
to  consist  of  particles,  these  having  perfect 
freedom  of  movement  in  gases,  more  res- 
trained powers  of  movement  in  liquids,  and 
comparatively  little  freedom  of  movement  in 
solids.  It  will  be  seen  subsequently  that 
these  minute  components  of  matter  which 
have  hitherto  been  defined  by  the  intention- 
ally vague  term  "  particle  "  must,  from  the 
chemical  point  of  view,  be  defined  with  much 
greater  precision.  The  broad  physical  con- 
ception of  matter  as  composed  of  discrete 
particles  has  been  translated  into  more 
concrete  terms  by  modern  Chemistry  with 
such  marvellous  success  that  we  may  be  said 
to  have  made  some  progress  towards  the 
realization  of  those  supernatural  powers  of 
vision  which  have  been  ascribed  to  our 
imaginary  being. 

Physical  Separation  of  the  Components  of 
Air. — The  transition  from  the  solid  to  the 
liquid,  or  from  the  liquid  to  the  gaseous  form 
of  matter,  or  the  reverse  series  of  changes,  are 
physical  phenomena  made  most  familiar  to 
us  in  the  case  of  water,  which  we  all  know  in 


THE    COMPONENTS    OF   AIR       59 

the  forms  of  ice,  water,  and  steam,  or  water 
vapour.  The  precise  temperatures  at  which 
these  changes  of  state  take  place  are  physico- 
chemical  characters  of  the  various  substances, 
and  are  dependent  upon  the  heat  imparted  to 
or  withdrawn  from  the  substance,  the  heat 
so  imparted  or  withdrawn  being  generally 
measured  on  the  scale  of  a  thermometer. 
Thus,  the  point  at  which  a  change  of  state 
takes  place  is  said  to  be  the  melting-point  or 
point  of  fusion  of  the  substance  if  it  passes 
from  the  solid  to  the  liquid  state,  or  the  freez- 
ing-point or  point  of  solidification  if  the  change 
is  from  the  liquid  to  the  solid  state.  The 
temperature  at  which  a  liquid  overcomes  the 
atmospheric  pressure  and  passes  suddenly 
into  the  state  of  vapour  is  known  as  the 
boiling-point.  The  measurement  of  tempera- 
ture, the  determination  of  melting-points 
and  boiling-points,  the  various  thermometers 
and  their  scales,  the  effects  of  pressure,  etc., 
are  all  dealt  with  in  books  on  Physics  and  on 
practical  Chemistry.  It  will  suffice  for  the 
present  to  state  that  in  Chemistry  the  Centi- 
grade scale  is  always  used,  and  on  this  scale 
the  zero-point  (0°)  is  the  freezing-point  of 
water,  and  the  boiling-point  of  water  under 
the  average  atmospheric  pressure  of  76  centi- 
metres is  marked  100°. 


60  CHEMISTRY 

Thus  the  description  of  the  various  states 
of  physical  aggregation  of  matter  is  made 
more  definite  when  we  are  enabled  to  asso- 
ciate with  each  substance  its  specific  physico- 
chemical  characters  of  melting-point  or  boiling- 
point.  We  say,  for  example,  that  above  100° 
water  exists  as  vapour,  and  at  0°  as  ice.  And 
what  is  true  for  water  is  true  for  other  liquids 
and  for  other  gases ;  for  it  is  now  known  that 
those  forms  of  matter  which,  like  nitrogen  or 
oxygen,  are  gaseous  at  ordinary  temperatures 
are  really  the  vapours  of  liquids  whose  boiling- 
points  are  so  low  that  they  are  never  reached 
under  any  natural  terrestrial  conditions.  In 
other  words,  we  should  have  to  cool  air  down 
to  a  temperature  of  more  than  190°  below  the 
freezing-point  of  water  in  order  that  its 
physical  state  might  be  changed  and  the 
gaseous  mixture  condensed  to  a  liquid.  Now, 
the  liquefaction  of  air  and  of  other  gaseous 
forms  of  matter  has  been  effected  on  the  large 
scale  in  recent  times — a  feat  that  must  be 
regarded  as  among  the  great  achievements 
of  modern  science.  The  principle  made  use 
of  is  essentially  physical,  and  cannot  be 
considered  in  detail  here.  All  that  need  be 
said  is  that  it  is  a  self-cooling  process,  for, 
when  a  highly  compressed  gas  is  allowed  to 
escape  suddenly  from  its  containing  vessel, 


THE    COMPONENTS    OF    AIR       61 

it  cools  itself  by  expansion,  and  the  cooled  air 
can  then  be  made  to  cool  another  lot  of 
escaping  gas,  and  so  on  by  a  summing-up  of 
coldness  in  a  continuously  cooling  cycle  until 
the  point  of  liquefaction  is  reached.  Air  is 
thus  obtained  as  a  limpid  liquid  not  unlike 
water  in  appearance. 

Starting,  then,  with  air  reduced  to  a  liquid 
in  some  suitable  apparatus  by  the  method 
described,  it  is  obvious  that,  when  such  a 
liquid  is  allowed  to  rise  in  temperature  by 
exposure  to  the  atmospheric  temperature, 
which  in  this  country  averages  about  200° 
above  the  boiling-point  of  liquid  air,  the  latter 
will  begin  to  boil,  and  so  resume  again  the 
gaseous  state.  But  if  the  oxygen  and  nitrogen 
of  the  air  are  not  chemically  combined,  then 
liquid  air  must  consist,  not  of  a  definite 
compound,  but  of  a  mixture  of  liquids  ;  and 
if  the  various  constituents  have  different 
boiling-points,  then,  as  the  liquid  air  passes 
into  gas,  it  might  be  expected  that  the 
constituent  which  passed  most  readily  into 
gas,  i.e.,  which  had  the  lowest  boiling-point, 
would  boil  off  more  rapidly  than  the  con- 
stituents which  had  the  higher  boiling-point. 

To  make  this  point  quite  clear,  the  reverse 
process  may  be  considered,  viz.,  the  cooling 
down  of  a  mixture  of  gases  to  the  point  of 


62  CHEMISTRY 

liquefaction.  In  this  case,  the  most  easily 
condensible  gas  would  be  that  which  had  the 
highest  boiling-point,  so  that  the  liquid 
obtained  from  such  a  mixture  would  be  richer 
in  that  component.  Now,  the  boiling-points 
of  the  main  constituents  of  the  air,  oxygen 
and  nitrogen,  are  about  183°  and  196°  below 
the  freezing-point  of  water  respectively  ;  in 
other  words,  nitrogen  boils  about  13°  lower 
than  oxygen.  Consequently,  on  the  assump- 
tion that  air  is  a  mixture  and  not  a  compound, 
the  mere  physical  act  of  liquefaction  might 
be  expected  to  upset  the  composition,  since 
the  oxygen  with  the  higher  boiling-point 
would  condense  more  readily  than  the 
nitrogen  with  the  lower  boiling-point.  This 
is  actually  found  to  be  the  case — the  gaseous 
air  recovered  from  liquid  air  contains  nearly 
double  the  quantity  of  oxygen  contained 
in  normal  air.  Moreover,  if  liquid  air  is 
allowed  to  boil  by  exposure  to  the  ordinary 
temperature,  the  nitrogen  boils  off  more 
rapidly  than  the  oxygen,  and  there  is 
finally  left  a  liquid  residue  which,  on  being 
allowed  to  gasify,  is  found  to  be  still  richer 
in  oxygen,  so  that  a  continuous  process  of 
separation  is  effected  without  calling  in  the 
agency  of  any  other  form  of  matter  capable  of 
removing  the  oxygen  or  the  nitrogen  by 


THE    COMPONENTS    OF    AIR       63 

chemical  combination.  It  is  a  case  of 
separation  by  purely  physical  means — a 
separation  which  would  have  been  impos- 
sible if  the  oxygen  and  nitrogen  had 
been  chemically  combined.  Thus  there  is 
furnished  another  proof  that  the  air  is  a 
mixture,  and  not  a  chemical  compound, 


CHAPTER   III 

CHEMICAL  CHANGE  IN  ITS  QUANTITATIVE 
ASPECT — THE  DEFINITENESS  OF  CHEMICAL 
CHANGE THE  CONSERVATION  OF  MASS- 
WATER  A  CHEMICAL  COMPOUND 

Chemical  Change  in  its  Quantitative  Aspect. — 
Air  has  been  selected  as  a  type  of  a  mechani- 
cal mixture  because  of  its  familiarity  as  the 
medium  in  which  we  live.  All  the  chemical 
processes  which  go  on  in  the  world  of  life, 
animal  and  vegetable,  and  which  are  mani- 
festations of  "  vitality,"  are  dependent  upon 
one  or  another  of  the  constituents  of  that 
gaseous  envelope  which  enwraps  our  globe. 
Thus,  animals  require  oxygen  and  plants 
carbon  dioxide,  and  both  forms  of  life  require 
nitrogen,  which  is  supplied  directly  or  in- 
directly from  the  atmosphere.  We  speak 
now  of  the  gaseous  components  of  the  air  as 
forms  of  matter  with  the  same  confidence  that 
we  refer  to  a  solid,  such  as  a  lump  of  iron, 
or  a  liquid  such  as  water,  as  forms  of  matter. 
The  only  consideration  that  affects  the 

64 


CHEMICAL    CHANGE  65 

chemist  in  this  case  is  the  purely  practical 
one  of  difference  in  methods  of  manipulation 
which,  when  dealing  with  gaseous  matter, 
at  first  presented  difficulties  not  met  with 
when  dealing  with  solids  or  liquids  ;  and  the 
early  history  of  our  science  will  be  found  to 
contain  most  instructive  records  of  the  groping 
after  the  truth  not  only  that  gases  were  to  be 
considered  material  substances,  but  that 
there  were  different  kinds  of  gases  or  "  airs  " 
as  distinct  from  one  another  as  iron  from 
chalk,  or  as  chalk  from  sulphur.  The  difficulty 
arose,  of  course,  from  the  circumstance  that 
gases  such  as  oxygen,  nitrogen,  and  carbon 
dioxide,  being  all  colourless,  transparent, 
and  invisible  substances,  do  not  directly 
reveal  to  our  senses  their  individual  characters 
or  specific  properties.  The  modern  student 
learns  at  a  very  early  period  to  distinguish 
between  such  gases,  a  jar  of  oxygen,  for 
example,  causing  a  splinter  of  glowing  wood 
thrust  into  the  gas  to  burst  at  once  into  flame 
owing  to  its  energetic  character  as  a  promoter 
of  combustion,  while  nitrogen  and  carbon 
dioxide  extinguish  flame.  The  latter  gas 
also  has  the  property  of  producing  turbidity 
in  a  clear  solution  of  lime  in  water,  which 
property  is  not  possessed  by  either  oxygen 
or  nitrogen.  And,  as  his  experience  increases, 


66  CHEMISTRY 

the  student  will  become  acquainted  with 
various  other  gases  to  which  diagnostic  tests 
can  be  applied,  some,  which  burn  in  the  air, 
being  said  to  be  combustible,  and  others 
possessing  characteristic  colours  and  odours, 
and  so  forth. 

Admitting,  therefore,  once  and  finally,  that 
the  physical  state  of  aggregation  of  matter 
is  of  secondary  importance  to  its  study  from 
the  chemical  point  of  view,  we  may  now  pass 
on  to  the  consideration  of  some  of  the  other 
characteristics  of  chemical  change.  In  many 
of  the  cases  already  referred  to,  such  as  the 
combination  of  iron  with  sulphur  or  with 
iodine — which,  it  will  be  remembered,  requires 
the  application  of  heat  in  the  case  of  sulphur, 
and  which  takes  place  spontaneously  in  the 
case  of  iodine — the  condition  laid  down  for 
the  production  of  a  new  and  homogeneous 
product  was  that  the  materials  should  be 
brought  together  in  certain  proportions  (p.  42). 
A  quantitative  notion  of  chemical  change  is 
thus  introduced  ;  and  with  this  notion  the 
student  of  Chemistry  must  become  thoroughly 
imbued.  When  chemical  change  occurs,  either 
as  the  result  of  combination,  or  of  decom- 
position, or  by  any  other  of  the  processes 
known  to  chemists,  it  is  found  that  the  actual 
quantities  of  matter  concerned  in  the  change 


CHEMICAL    CHANGE  67 

bear  a  definite  relationship  towards  each 
other.  Herein  will  be  found  one  of  the  prime 
characteristics  of  chemical  change  as  dis- 
tinguished from  mechanical  mixture.  It  is 
obvious  that,  in  a  mixture,  there  is  no  such 
restriction  with  respect  to  quantities.  Gases, 
liquids  that  are  miscible,  and  solid  powders 
may  be  mixed  together  in  any  proportions, 
and  the  product  partakes  of  the  character 
of  its  components.  The  chemist  is  concerned, 
therefore,  not  only  with  the  quality,  i.e., 
the  specific  properties  of  the  substances,  but 
also  with  the  relative  quantities  of  the 
materials  which  undergo  transformation. 

In  physical  terms,  quantity  of  matter  means 
mass  ;  and  for  practical  purposes  the  most 
convenient  measure  of  mass  is  the  gravi- 
tational measure — weight.  In  dealing  with 
gases,  and,  under  certain  circumstances, 
also  with  liquids,  the  bulk  or  volume  is 
a  convenient  measure ;  but  in  such  cases 
the  volume  measure  is  generally  for  ultimate 
purposes  expressed  in  terms  of  weight.  The 
reader  who  studies  the  history  of  Chemistry 
will  learn  that  our  science  only  began  to  take 
rank  as  an  exact  science  from  the  time  when 
chemical  change  was  studied  quantitatively 
by  means  of  that  delicate  weighing  machine 
known  as  the  chemical  balance. 


68  CHEMISTRY 

The  process  of  weighing,  the  description  of 
the  balance,  and  of  the  standards  of  weight 
in  use,  are  all  dealt  with  in  works  on  practical 
Physics  and  practical  Chemistry;  and  every 
student  is  now  familiarized  with  weighing 
operations  at  the  outset  of  his  practical  work. 
The  chief  points  which  the  general  reader 
must  realize  are  that  the  weighings  carried 
out  by  the  chemist  are  with  smaller  quantities, 
and  with  a  degree  of  precision  unknown  to 
those  who  are  accustomed  only  to  the  com- 
paratively rough  scales  or  weighing  machines 
in  ordinary  use.  For  scientific  purposes, 
also,  the  metric  system  of  weights  and 
measures  is  universally  employed.  In  this 
system  the  standard  of  length  is  the  metre, 
equal  to  39-37  inches,  and  the  unit  of  weight 
is  the  gram,  which  is  the  thousandth  part 
of  a  standard  mass  of  metal  (platinum)  kept 
in  Paris,  and  known  as  the  kilogram.  The 
kilogram  of  pure  water  at  its  maximum 
density  of  4°  Centigrade  occupies  a  volume  of 
1000  cubic  centimetres  (one  litre),  so  that 
the  unit  weight  of  one  gram  is  the  weight  of 
one  cubic  centimetre  of  pure  water  at  4°. 
Those  who  are  familiar  only  with  our  cumbrous 
English  system  of  weights  and  measures  will 
acquire  more  definite  notions  when  it  is 
pointed  out  that  the  gram  is  equal  to  15-43 


CHEMICAL    CHANGE  69 

grains,  the  kilogram  to  2-2  pounds,  and  the 
litre  to  61-03  cubic  inches  =  0-22  gallon. 
In  ordinary  quantitative  chemical  work, 
we  seldom  deal  with  quantities  exceeding 
100  grams — generally  with  much  less,  and  a 
good  chemical  balance  will  weigh  accurately 
to  l-10,000th  of  a  gram.  Such  refinement  in 
the  process  of  weighing  would  have  appeared 
incredible  to  the  early  pioneers  of  quantitative 
chemistry,  whose  most  accurate  operations 
in  the  eighteenth  century  were  carried  out 
with  balances  which  would  now  be  regarded 
as  relatively  coarse. 

The  Definiteness  of  Chemical  Change. — By 
means  of  the  balance  the  results  of  chemical 
transformation  can,  therefore,  be  followed 
quantitatively.  In  stating  that  the  material  or 
materials  which  undergo  such  change  bear  a 
definite  quantitative  relationship  towards  each 
other,  it  is  meant  that,  when  a  new  substance 
is  formed  or  when  substances  arise  from  the 
combination  or  from  the  decomposition  of 
other  kinds  of  matter,  the  relative  weights, 
both  of  the  original  materials  which  undergo 
transformation  and  of  the  new  product  or  pro- 
ducts, are  always  constant  for  each  particular 
kind  of  matter.  Since  combination  and  de- 
composition are  generally  reversible  processes, 
it  will  be  simpler  at  this  stage  to  consider 


70  CHEMISTRY 

combination  only.  From  this  point  of  view 
the  actual  weight  of  one  substance  which  can 
combine  chemically  with  another  substance 
to  produce  a  different  kind  of  matter  is  as 
much  a  specific  chemical  property  of  the 
substance  as  any  other  character.  The  only 
qualification  that  must  be  borne  in  mind  is 
that  different  substances  may  have  the 
property  of  combining  with  each  other  in 
more  than  one  proportion  by  weight ;  but 
this  fact  does  not  contravene  the  principle  of 
definite  combination — it  only  enlarges  the 
definition  of  the  principle  so  as  to  include 
more  than  one  possibility. 

An  example  illustrative  of  the  foregoing 
statement  has  already  been  furnished  in  the 
case  of  iron  (p.  23).  It  will  be  remembered 
that  this  substance  forms  "  scale  "  when  the 
heated  metal  is  acted  upon  by  water  or  air, 
and  rust  when  it  is  exposed  to  moist  air  at 
ordinary  temperatures.  Now,  iron  "  scale  " 
and  iron  rust  (when  dry)  consist  of  the  same 
two  substances,  iron  and  oxygen,  so  that  we 
have  here  two  dissimilar  compounds  arising 
from  the  combination  of  the  same  two 
materials.  There  is,  of  course,  no  mystery 
about  this  ;  the  balance  can  be  made  to  prove 
that  the  difference  is  due  to  the  fact  that, 
in  "  scale,"  the  iron  and  oxygen  are  present  in 


CHEMICAL    CHANGE  71 

different  proportions  from  those  in  which  they 
are  present  in  rust.  And,  in  order  to  render 
the  story  more  complete,  it  may  be  mentioned 
that  we  know  a  third  compound  containing 
only  iron  and  oxygen  which  is  easily  obtainable 
by  chemical  methods,  and  which  is  quite 
distinct  as  a  form  of  matter  from  both  scale 
and  rust,  and  in  which  the  proportions  of  iron 
and  oxygen  are  again  different.  Since  com- 
pounds formed  by  the  combination  of  oxygen 
with  other  substances  are  generally  known 
in  Chemistry  as  oxides,  the  case  will  be  made 
more  definite  by  stating  that  there  exist 
three  oxides  of  iron — three  different  sub- 
stances all  arising  from  the  combination  of 
the  same  two  materials  in  different  pro- 
portions. It  is  quite  easy  to  ascertain  by 
methods  which  must  be  considered  hereafter 
the  relative  proportions  of  iron  and  oxygen 
contained  in  the  three  oxides  ;  and  it  will  give 
greater  precision  to  the  notion  of  chemical 
transformation  in  its  quantitative  aspect  if 
it  is  stated  that  the  iron  oxide  of  rust  contains 
70  per  cent,  of  iron,  the  iron  oxide  of  "  scale  " 
72-4  per  cent.,  and  the  third  oxide  77-8  per 
cent,  of  iron. 

The  main  principle — the  definiteness  of 
chemical  change — is  thus  upheld,  because 
whenever  iron  is  made  to  combine  with 


72  CHEMISTRY 

oxygen,  directly  or  indirectly,  we  always 
get  one  or  another  of  these  three  substances. 
There  are  no  definite  intermediate  substances ; 
the  defmiteness  is  chemically  rigid.  It  is  a 
chemical  truth  expressed  in  numerical  terms 
that  cannot  be  tampered  with.  If  it  were 
desired  to  produce  a  substance  containing 
iron  and  oxygen  in  some  other  proportions 
than  those  indicated,  it  could  not  be  done 
by  chemical  means,  but  only  by  making  a 
mechanical  mixture  of  some  or  all  of  the  three 
oxides  to  each  of  which  Nature  has  attached 
the  brand  of  individuality. 

It  will  now  be  understood  why  the  pro- 
duction of  a  definite  homogeneous  substance 
by  chemical  change  is  conditioned  by  the 
relative  quantities  of  the  materials  brought 
into  combination.  The  relative  weights  of 
the  materials  which  combine  being  fixed  in 
the  sense  indicated,  if  too  much  or  too  little 
of  one  of  the  components  is  used  the  product 
of  the  transformation  is  necessarily  a  mixture 
consisting  of  the  new  compound,  i.e.,  the 
chemical  product,  plus  the  excess  of  unchanged 
material.  For  example,  the  compound  formed 
by  heating  iron  with  sulphur  (p.  39)  is  a 
sulphide  (note  the  analogy  of  the  term  with 
oxide)  of  iron  containing  iron  and  sulphur 
in  the  ratio  4:3.  If  more  iron  is  put  into 


CHEMICAL    CHANGE  73 

the  original  mixture,  the  product  would 
consist  of  the  sulphide  mixed  with  the  excess 
of  unconverted  iron.  Again,  the  iron  iodide 
formed  by  the  direct  union  of  iron  with 
iodine  (p.  38)  is  a  product  resulting  from 
the  combination  of  7  parts  by  weight 
of  iron  with  31.7  parts  of  iodine.  These 
materials  cannot  be  made  to  combine  in  the 
chemical  sense  in  any  other  proportions ; 
any  excess  either  of  iron  or  of  iodine  would 
be  left  uncombined,  and  could  be  separated 
from  the  mixture  by  appropriate  methods. 

The  very  definiteness  of  chemical  change 
thus  prohibits  a  gradual  transition  from  one 
form  of  matter  to  another.  There  is  neces- 
sarily an  abruptness  about  the  transforma- 
tions ;  there  can  be  no  half  measures — 
each  material  must  have  its  full  complement 
of  its  associated  material.  If  it  is  not  supplied 
with  as  much  as  it  can  combine  with,  there 
is  a  residue  of  unappropriated  and  unchanged 
material.  In  speaking  of  chemical  change 
as  abrupt,  it  is  not  meant  that  the  transforma- 
tion takes  place  suddenly.  There  are  chemical 
changes  of  every  degree  of  velocity,  from  a 
practically  instantaneous  explosion  to  a  trans- 
formation which  may  take  days,  weeks,  years, 
or  geological  periods.  Direct  combination  is 
generally,  but  not  invariably,  a  rapid  process. 


74  CHEMISTRY 

But,  whether  rapid  or  slow,  the  definite 
character  of  chemical  change  is  universally 
maintained.  Indeed,  the  measurement  of 
the  velocity  or  rate  of  chemical  change  is 
only  made  possible  in  those  cases  where  it  is 
measurable  by  the  definiteness  of  the  result, 
because  the  quantity  of  chemically  trans- 
formed material  (as  distinguished  from  the 
unchanged  material)  which  is  produced  at 
measured  intervals  of  time  can  be  determined 
either  directly  or  indirectly  by  means  of  the 
balance. 

Some  of  these  points  are  illustrated  by  the 
examples  of  chemical  change  already  cited. 
Thus,  the  rusting  of  iron  is  a  slow  process  ; 
the  conversion  of  iron  into  "  scale  "  by  the 
action  of  water  or  oxygen  upon  the  heated 
metal  is  a  sudden  change.  When  a  mixture 
of  iron  and  sulphur  is  heated  to  the  point 
at  which  combination  takes  place,  the  whole 
mass  suddenly  glows,  and  the  conversion  into 
sulphide  is  so  rapid  that  it  may  practically 
be  considered  to  be  instantaneous.  In  some 
cases,  both  of  slow  and  of  rapid  change,  it  has 
been  observed  that  the  final  stage  is  reached 
through  a  series  of  intermediate  steps,  but 
the  definiteness  of  the  transformation  is  not 
in  any  way  interfered  with — each  stage  is 
as  definite  as  its  successor ;  it  is  simply  a 


CHEMICAL    CHANGE  75 

case  of  a  definite,  stable,  final  stage  being 
reached  through  one  or  more  unstable  but 
equally  definite  intermediate  stages.  In  some 
cases  it  is  possible  to  arrest  the  process  at 
an  intermediate  stage,  and  to  isolate  the 
intermediate  product.  The  latter  is  in  all 
cases  the  result  of  combination  (or  of  decom- 
position) in  as  definite  proportions  by  weight 
as  is  the  final  product — the  end  result.  In 
other  words,  the  course  of  the  chemical 
change  can  in  many  cases  be  followed  through 
successive  phases,  each  perfectly  definite ; 
and  it  has  accordingly  been  suggested  with 
much  plausibility  that  all  chemical  change 
takes  place  in  stages,  these  stages  in  the 
majority  of  cases  being  passed  through  too 
rapidly  to  enable  them  to  be  detected  by  our 
present  methods.  Chemical  change  has  thus 
been  aptly  compared  to  a  drama,  the  scenes 
of  which  are  shifted  with  great  rapidity,  the 
spectator  seeing  only  the  final  act.  Applying 
this  metaphor  to  the  present  discussion,  the 
main  point  to  be  borne  in  mind  is  that 
each  scene  in  the  drama — even  if  passed 
through  with  explosive  velocity — is  a  perfectly 
definite  picture  from  the  chemical  point  of 
view,  although  the  eye  of  the  chemist  may 
not  at  present  be  capable  of  appreciating  it. 
A  great  step  in  the  development  of  the 


76  CHEMISTRY 

science  of  Chemistry  will  have  been  made 
when  new  methods  are  discovered  or  known 
methods  applied  to  the  study  of  what  may 
be  called  transition  phases. 

The  Conservation  of  Mass. — It  will  have 
been  made  evident  to  the  reader  that,  although 
chemical  change  is  characterised  by  those 
general  criteria  which  have  been  discussed, 
there  is  still  an  individuality  about  the 
process  which  makes  it  essential  both  for 
scientific  and  for  practical  purposes  to  study 
particular  cases  in  detail.  It  is,  in  fact,  by 
such  detailed  studies  that  our  generalized 
statements  have  been  made  possible.  The 
vastness  of  the  prospect  thus  opened  out  to 
the  gaze  of  the  student  of  Chemistry  will  now 
begin  to  loom.  Before  proceeding  to  further 
developments,  however,  there  yet  remains  an 
aspect  of  chemical  change  of  fundamental 
importance  which  the  student  will  be  brought 
to  face  at  the  very  beginning  of  his  studies. 
It  will  have  been  realized  that  Chemistry  is 
concerned  with  the  transformations  of  matter, 
and  that  these  transformations  are,  so  far  as 
concerns  the  relative  masses  of  the  materials 
which  undergo  transformation,  of  a  perfectly 
definite  character.  It  is  but  another  step  to 
the  conclusion — based,  of  course,  upon  experi- 
mental evidence — that  during  such  trans- 


CONSERVATION   OF   MASS         77 

formations  there  is  neither  gain  or  loss  of 
material.  This  is  tantamount  to  the  proposi- 
tion that  matter  is  indestructible  by  any  known 
chemical  process.  There  is  profound  modifi- 
cation or  transformation  as  the  result  of 
chemical  change,  but  the  whole  quantity  of 
matter,  i.e.,  the  mass,  remains  constant ;  the 
sum  total  of  the  weights  of  the  materials 
employed  is  the  same  at  the  end  of  the 
transformation  as  it  was  before  the  change 
occurred.  Seven  unit  weights  of  iron  heated 
with  four  unit  weights  of  sulphur  give  eleven 
unit  weights  of  iron  sulphide  with  the  arith- 
metical accuracy  expressed  by  7+  4=  11,  and 
so  on  for  all  other  cases  of  chemical  change, 
either  by  combination  or  by  decomposition. 
If,  for  example,  the  above  process  could  be 
reversed  (as  it  can  by  indirect  methods),  and 
the  iron  sulphide  resolved  into  its  components, 
eleven  parts  of  the  sulphide  would  give  seven 
parts  of  iron  and  four  of  sulphur  or,  arith- 
metically, 11  =  7+4. 

The  statement  that  you  only  get  out  of  a 
given  weight  of  matter  as  much  as  you  start 
with  may  appear  at  first  sight  such  a  self- 
evident  proposition  that  no  special  proof  need 
be  adduced.  But  there  is  a  deeper  significance 
in  the  statement  than  might  be  imagined  on 
superficial  consideration.  In  the  first  place, 


78  CHEMISTRY 

it -involves  the  great  principle  that  matter  can 
neither  be  created  nor  destroyed  by  chemical 
agencies.  In  the  next  place,  it  opposes  what 
might  be  considered  the  common-sense  view 
that  such  deep  seated  and  startling  trans- 
formations as  are  wrought  by  chemical 
change  must  necessarily  be  accompanied  by 
loss  or  gain  of  material.  That  such  a  view 
should  have  been  entertained  in  former  times 
is  not  surprising  when  it  is  remembered  that 
the  truth  of  the  doctrine  can  only  be  de- 
monstrated when  all  the  products  of  the 
change  are  collected  and  weighed.  If,  as  is 
often  the  case,  one  of  the  products  is  gaseous 
and  escapes  in  an  invisible  form,  it  is  under- 
standable that,  at  a  time  when  the  material 
nature  of  gases  had  not  been  thoroughly 
recognized,  it  should  have  been  believed  that 
chemical  change  might  be  accompanied  by  a 
destruction  of  matter.  Although  the  scope 
of  this  work  does  not  admit  of  the  historical 
treatment  of  the  subject,  it  must  be  recognized 
as  marking  an  epoch  in  the  history  of  Chemis- 
try, that  the  conversion  of  the  scientific  world 
to  the  doctrine  of  the  indestructibility  of 
matter  was  brought  about  mainly  by  the 
researches  of  the  illustrious  French  chemist 
Lavoisier  (1743-1794),  who  fell  a  victim  to 
the  Revolution.  The  student  of  the  history 


CONSERVATION    OF    MASS         79 

of  our  science  will  learn  that,  although  the 
materials  for  the  establishment  of  this  doctrine 
were  in  hand,  its  acceptance  had  been 
retarded  by  the  prevalence  of  certain  erroneous 
theoretical  views  which  were  only  finally 
overthrown  by  the  experiments  and  reasoning 
of  Lavoisier  and  his  disciples. 

A  principle  so  fundamental  as  that  of  the 
indestructibility  of  matter,  which  now  per- 
meates every  branch  of  science,  must  enter 
into  the  mental  constitution  of  the  would-be 
chemist.  Through  every  phase  of  chemical 
change  it  is  known  that  this  principle  holds 
good,  although  in  ordinary  practical  work 
it  is  never  realized  unless  the  most  refined 
methods  are  employed  for  ascertaining  the 
weights  of  all  the  products.  In  every-day 
laboratory  or  factory  experience,  there  is 
always  more  or  less  loss  due  to  the  imperfec- 
tion of  methods  ;  and  the  theoretical  ideal 
that  the  sum  total  of  the  weights  of  the  final 
product  or  products  is  equal  to  the  sum  total 
of  the  weights  of  the  initial  materials  is 
seldom  realized.  But  the  loss  arising  from  un- 
avoidable manipulative  causes  no  longer  shakes 
faith  in  the  doctrine,  although,  for  certain 
philosophical  reasons,  it  has  been  thought 
necessary  to  retest  its  truth  with  the  utmost 
refinement  of  modern  scientific  resources. 


80  CHEMISTRY 

Water  a  Chemical  Compound. — In  the  light 
of  these  principles — the  definiteness  of  chemi- 
cal change,  and  the  indestructibility  of  matter 
— all  chemical  transformations  may  be  con- 
sidered and  illustrative  examples  multiplied 
indefinitely.  One  other  case  may  be  introduced 
at  this  stage,  both  on  account  of  its  historical 
interest  and  because  it  will  enable  fresh 
materials  to  be  added  to  the  small  store  of  facts 
which  have  thus  far  been  found  to  furnish  a 
sufficient  basis  for  the  discussion  of  the  broader 
generalities.  Attention  is  invited  to  another 
very  familiar  substance,  water,  which,  since  it 
forms  iron  scale  when  brought  into  contact 
with  the  heated  metal  (p.  23),  may  be  con- 
sidered to  have  been  proved  to  be  a  substance 
containing  oxygen.  Tlie  question  whether 
the  oxygen  in  air  and  water  may  not  -be 
present  in  a  different  condition  in  the  two 
substances  has  already  been  raised  (p.  27). 
Air  has  been  proved  to  be  a  mixture  ;  water 
has  been  proved  to  contain  oxygen,  and, 
since  water  differs  so  fundamentally  from 
oxygen  in  all  its  properties,  chemical  and 
physical,  it  is  evident  that  it  must  contain 
something  in  addition  to  oxygen.  What  is 
that  other  constituent,  or  what  are  the  other 
constituents  if  more  than  one;  and  is  the 
oxygen  simply  mixed  with  the  other  con- 


COMPOSITION    OF    WATER        81 

stituent  or  constituents,  or  is  it  chemically 
combined  ?  The  answer  to  these  questions 
will  furnish  further  instructive  illustrations  of 
facts,  methods,  and  principles. 

The  fact  that  heated  iron  takes  oxygen  out 
of  water  suggests  the  use  of  this  metal  in  order 
to  find  out  what  is  left  after  the  oxygen 
has  been  removed.  In  forming  iron  scale  by 
plunging  red-hot  iron  into  water,  it  might  be 
noticed  that  with  the  escaping  steam  there 
is  a  gas  which,  unlike  the  steam,  does  not 
condense  on  cooling.  This  gaseous  product 
of  the  action  of  water  upon  hot  iron  is  not 
easy  to  detect  by  such  a  rough  and  ready 
experiment ;  but  by  a  refinement  of  method, 
and  without  in  any  way  interfering  with  the 
principle  by  altering  the  materials,  this  gas 
can  be  readily  obtained  in  any  desired 
quantity.  In  practice,  instead  of  using  water, 
we  use  steam,  which  is  the  same  substance  in 
a  different  state  of  physical  aggregation  ;  and 
the  iron,  in  coarse  fragments,  such  as  wire 
or  nails,  is  enclosed  in  a  tube  of  porcelain 
or  any  suitable  material  that  will  stand  the 
heat.  The  steam  is  passed  through  the  heated 
tube,  and  the  emergent  gas  collected  by 
appropriate  methods  in  any  suitable  vessel. 
At  the  end  of  such  an  operation,  the  iron  will 
be  found  to  have  been  more  or  less  converted 

F 


82  CHEMISTRY 

into  scale ;  and  the  new  gas,  on  comparison 
with  oxygen  or  nitrogen,  will  be  found  to 
possess  quite  different  properties — it  is  a 
different  kind  of  gaseous  matter.  This  gas  is 
known  as  hydrogen  ;  like  nitrogen  or  carbon 
dioxide  (p.  36),  it  is  colourless  and  transparent, 
and  it  extinguishes  flame,  but,  unlike  these 
gases,  it  is  combustible,  burning  in  the  air 
with  a  barely  perceptible,  but  very  hot  flame. 
Hydrogen  is  the  lightest  form  of  matter  known 
on  this  earth,  being,  bulk  for  bulk,  14 J  times 
lighter  than  air,  16  times  lighter  than  oxygen, 
and  11,000  times  lighter  than  water.  In 
view  of  this  fact,  the  gas  is  difficult  to  deal 
with  practically,  since  it  tends  to  escape 
from  all  vessels,  tubes,  joints,  etc.,  having  the 
slightest  porosity. 

Water  is  thus  shown  to  contain  hydrogen 
as  well  as  oxygen  ;  and,  since  by  no  physical 
process,  such  as  diffusion  (p.  56),  is  it  possible 
to  separate  hydrogen  from  water,  it  is  evident 
that  the  gas  is  chemically  combined.  The 
question  whether  the  oxygen  in  air  and  water 
may  not  be  present  in  different  states  is  thus 
definitely  answered.  Moreover,  a  mixture  of 
hydrogen  with  oxygen  is  not  water,  but  a 
gas  intermediate  in  properties  between  it?' 
two  components.  The  oxygen  in  water  i/n 
chemically  combined,  and  its  separation  by 


COMPOSITION    OF    WATER        83 

hot  iron  is  a  chemical  process.  It  will  be 
noticed  that  the  removal  of  oxygen  from  water 
by  iron  differs  materially  as  a  process  from 
that  which  takes  place  when  iron  rusts  in  air  : 
in  the  first  case  the  iron  has  to  be  red-hot, 
while  in  the  second  case  the  combination 
takes  place  at  the  ordinary  temperature. 
Chemical  combination  has  to  be  overcome 
in  the  case  of  water,  and  not  in  the  case  of 
air,  which  is  simply  a  mixture. 

From  this  illustration  we  can  develop 
further  the  principle  of  preferential  combina- 
tion (p.  35),  because  there  are  other  metals 
besides  iron  which  liberate  hydrogen  from 
water,  while  many  other  metals  exert  no  such 
decomposing  action.  Thus,  a  familiar  metal, 
zinc,  when  heated  in  steam  liberates  hydrogen 
at  a  lower  temperature  than  iron,  and  another 
less  familiar  metal,  magnesium,  at  a  still 
lower  temperature ;  while  some  metals,  which 
are  not  generally  familiar,  and  which  will  be 
afterwards  referred  to,  will  decompose  even 
liquid  water  and  liberate  hydrogen  at  the 
ordinary  temperature.  So  we  could  con- 
struct a  graduated  scale  of  preferences 
for  oxygen  as  measured  by  the  temperature 
at  which  the  hydrogen  is  liberated  from 
water.  On  such  a  scale  the  inverse  prefer- 
ential order  would  be  iron,  zinc,  magnesium, 


84  CHEMISTRY 

and  then  those  metals  which  decompose  cold 
water. 

Then  again,  as  already  stated,  some  metals 
do  not  liberate  hydrogen  from  water  at  all, 
although  they  are  capable  of  combining  with 
oxygen.  Such  a  metal  is  the  familiar  sub- 
stance copper,  which,  like  iron,  when  heated 
unites  with  oxygen  to  form  an  oxide,  a  black 
substance  totally  unlike  the  red  lustrous 
metal  and  the  gaseous  oxygen  from  which 
it  is  formed.  The  removal  of  oxygen  from 
the  air  by  passing  the  latter  through  a  tube 
containing  heated  copper  is  one  of  the 
recognized  methods  of  obtaining  nitrogen. 
These  facts  furnish  further  proof  of  water 
being  a  chemical  compound,  because  one 
metal  (iron),  which  has  a  sufficiently  strong 
liking  for  oxygen  to  combine  with  this 
substance  at  the  ordinary  temperature,  has  to 
be  made  red-hot  before  it  can  take  the  oxygen 
out  of  water,  while  another  metal  (copper), 
which  has  sufficient  liking  for  oxygen  to 
form  an  oxide  when  heated  in  contact  with 
that  gas,  fails  to  remove  the  oxygen  from 
water  even  when  red-hot.  From  which  facts 
it  follows,  also,  that  on  the  inverted  scale  of 
preferences  for  oxygen  given  above,  copper 
would  precede  iron. 

And  now  there  remains   to   be  answered 


COMPOSITION    OF    WATER        85 

the  question  whether  water  contains  anything 
besides  oxygen — a  question  which,  of  course, 
relates  only  to  water  as  an  individual  form  of 
matter,  and  not  to  water  as  we  find  it  in 
nature,  in  rivers,  or  in  rain,  or  in  the  sea, 
because  such  water  always  contains  other 
substances  dissolved  in  it.  Natural  water  is, 
in  fact,  an  aqueous  solution  of  substances 
derived  from  the  air  and  the  earth  through 
which  it  percolates.  When  a  liquid  like 
water  takes  up  other  substances,  either  solid, 
liquid,  or  gaseous,  and  forms  a  homogeneous 
mixture,  the  latter  is  technically  described 
as  a  solution  ;  the  water  in  this  case  is  des- 
cribed as  the  solvent  and  the  other  (dissolved) 
substances  as  the  solutes.  This  point  is  raised 
in  a  preliminary  way  now,  in  order  that 
it  may  be  realized  that  we  are  at  present 
concerned  only  with  water,  and  not  with  any 
adventitious  matter  that  it  may  contain. 
Returning  now  to  the  initial  question,  and 
referring  to  the  statement  that  a  mixture 
of  oxygen  and  hydrogen  is  not  water,  it  only 
remains  to  be  added  that  a  mixture  of  these 
gases  explodes  when  a  flame  is  applied, 
and  the  product  has  been  found  to  be  water 
and  nothing  but  water.  This  is  an  old  and 
now  a  well  known  fact,  but  the  reader  must 
again  be  reminded  that  a  discovery  of  this 


86  CHEMISTRY 

importance  was  made  only  by  skilful  experi- 
ment carried  out  with  all  the  refinements  of 
method  available  at  the  time  by  Cavendish 
(1731-1810).  In  practice,  the  gases  are  en- 
closed in  a  strong  glass  vessel  (eudiometer) 
with  platinum  wires  sealed  through  the  side, 
so  that  the  explosive  combination  may  be 
brought  about  by  passing  an  electric  spark 
through  the  mixture. 

Since  oxygen  and  hydrogen  give  nothing 
but  water  when  combined,  the  question  is 
therefore  answered — water  as  such  contains 
no  other  form  of  matter.  So  that,  as  the 
result  of  chemical  transformation,  a  gas, 
hydrogen,  of  which  the  boiling-point  is  about 
252°  below  the  freezing-point  of  water,  in 
combination  with  another  gas,  oxygen,  of 
which  the  boiling-point  is  about  183°  below 
the  freezing-point  of  water,  gives  a  liquid 
1,400  times  denser  than  hydrogen  and  boiling 
at  100°  above  the  freezing-point  of  water. 
The  profound  nature  of  chemical  change  is 
again  exemplified ;  but  its  definiteness  has 
not  yet  been  made  apparent  in  the  case  under 
consideration.  Supposing,  then,  that  into  a 
suitable  vessel,  such  as  a  strong  glass  tube 
graduated  by  marked  divisions  which  enable 
us  to  measure  the  volume  of  gas,  and  having 
wires  sealed  through  to  enable  an  electric 


COMPOSITION    OF    WATER        87 

spark  to  be  passed,  we  introduce  hydrogen 
and  oxygen  in  known  volumes.  It  is  not  easy 
— speaking  strictly  it  is  impossible — to  form 
an  accurate  notion  of  the  method  of  conduct- 
ing such  an  experiment  as  this  by  simply 
reading  a  description  of  it.  The  student  of 
Chemistry,  however,  learns  how  to  manipulate 
gaseous  matter  with  the  same  facility  that 
he  deals  with  solids  or  liquids,  and  the  methods 
are  given  in  practical  works  on  the  subject. 
But  the  principle  is  quite  intelligible,  for  we 
have  only  to  consider  that  we  have  a  mixture 
of  hydrogen  and  oxygen  in  which  the  actual 
volume  of  each  gas  is  known.  The  mixture 
is  exploded  by  an  electric  spark,  and  some  of 
the  gas  disappears  when  the  vessel  has  cooled 
down.  In  other  words,  there  is  shrinkage  of 
volume,  because  the  water  which  is  formed, 
being  liquid  at  ordinary  temperatures,  con- 
denses to  a  dew  on  the  inside  of  the  vessel. 
But  the  whole  of  the  gas  does  not  disappear 
in  an  experiment  of  this  kind  ;  there  is 
always  a  residue,  and,  by  testing  that  residue, 
it  is  easily  shown  that  it  is  either  oxygen  or 
hydrogen,  according  to  the  proportions  of  the 
gases  in  the  original  mixture.  By  such  obser- 
vations it  is  found  that,  for  every  unit  volume 
of  oxygen,  two  unit  volumes  of  hydrogen 
disappear,  or  conversely,  that  for  every  unit 


88  CHEMISTRY 

of  hydrogen  half  a  unit  volume  of  oxygen 
disappears. 

Hereby  is  once  more  illustrated  the  definite- 
ness  of  chemical  change  ;  under  the  conditions 
imposed,  the  gases  refuse  to  combine  in  any 
other  proportions — there  are  no  intermediate 
proportions  ;  it  is  two  to  one,  or  nothing. 
Any  excess  of  either  gas  over  and  above  these 
proportions  is  left  uncombined,  and  that  is 
why  in  such  experiments  there  is  always 
some  residual  gas  ;  it  is  only  when  a  mixture 
of  exactly  two  volumes  of  hydrogen  with  one 
volume  of  oxygen  is  exploded  that  there  is 
no  gas  left — the  sole  result  of  the  combination 
is  then  water,  and  the  vessel  in  which  the 
explosion  takes  place  is  found  to  be  practically 
vacuous  when  cold,  because  the  condensed 
water  occupies  but  a  very  small  volume.  The 
defmiteness  in  this  case  manifests  itself  volu- 
metrically,  but  that  is  really  the  same  thing  as 
defmiteness  by  weight,  because  the  gases 
have  perfectly  definite  weights  as  compared 
with  each  other  under  comparable  conditions, 
the  relative  weights  of  equal  volumes  of 
oxygen  and  hydrogen  being  about  16 :  1. 
As  one  volume  of  oxygen  combines  with 
two  volumes  of  hydrogen,  the  relative  weights 
of  the  two  substances  contained  in  water  are 
accordingly  16  :  2  =8:1. 


COMPOSITION    OF    WATER        89 

Other  methods  of  determining  the  com- 
position of  water  are  well  known,  but  these 
involve  new  principles,  and  cannot  be  dis- 
cussed now.  The  chemical  change  which 
takes  place  when  hydrogen  and  oxygen 
combine  is,  like  all  other  chemical  changes, 
a  redistribution  or  rearrangement  of  matter, 
but  the  weight  of  the  materials  remains 
constant — the  principle  of  the  Conservation 
of  Mass  is  maintained.  If  the  vessel  contain- 
ing the  gases  is  weighed  before  and  after  the 
explosion,  there  will  be  found  neither  loss  nor 
gain — a  fact  which  in  this  instance  is  made 
more  striking  because  of  the  total  disappear- 
ance of  the  gases  as  such  when  the  mixture 
contains  the  correct  proportions. 

The  principle  of  the  Conservation  of  Mass, 
which  lies  at  the  root  of  the  science  of  Chemis- 
try, will  remind  the  student  of  Physics  of  the 
analogous  principle  of  the  Conservation  of 
Energy.  To  the  chemist,  this  last  doctrine 
is  of  equal  importance  with  the  first,  but  the 
reader  must  beware  of  straining  the  analogy 
between  the  two  doctrines.  The  different 
forms  of  energy  are  interconvertible,  but  it 
by  no  means  follows  that  different  forms  or 
kinds  of  matter  are  interconvertible.  Were 
such  interconvertibility  possible,  we  should 
have  realized  "  transmutation "  as  distin- 


90  CHEMISTRY 

guished  from  transformation.  Transmuta- 
tion was  the  moving  principle  prompting 
the  work  of  the  old  alchemists,  gold  being 
the  form  of  matter  striven  for.  It  is  now 
recognized  that  alchemy  failed  in  this  particu- 
lar quest,  and  the  idea  of  transmutation 
passed  out  of  Chemistry  when  it  became  a 
science.  In  modern  times,  and  in  the  light 
of  new  discoveries,  the  idea  has  been  rein- 
stated in  another  form,  and  there  is  some 
evidence  in  its  favour — how  much  is  a  matter 
of  judgment  and  a  question  for  future  investi- 
gation. But  all  this  belongs  to  another  story 
which  cannot  be  narrated  here,  and  the 
reader  must  be  referred  for  further  information 
to  works  dealing  with  Radioactivity,  or  to 
the  volume  on  "  Matter  and  Energy "  by 
Mr.  F.  Soddy,  in  this  series. 


CHAPTER  IV 

ELEMENTARY  AND  COMPOUND  MATTER — THE 
CHEMICAL  ELEMENTS — METALS  AND  NON- 
METALS 

Elementary  and  Compound  Matter. — Matter  is 
comprised  under  the  general  term  "  stuff," 
an  expression  which  finds  its  equivalent  in 
the  German  Stoff.  Thus  we  speak  of  food- 
stuffs, dyestuffs,  etc.,  and  the  German  for 
hydrogen,  Wasserstoff,  indicates,  like  the 
Greek  roots  of  the  English  name  for  this 
substance,  that  it  is  the  stuff  from  which  water 
is  produced.  For  the  purpose  of  illustrating 
the  nature  of  chemical  change,  appeal  was 
made  in  the  preceding  chapters  to  a  number 
of  well-known  things,  such  as  air  and  water, 
iron,  sulphur,  iodine,  sugar,  charcoal,  and 
chalk.  By  the  chemical  study  of  some  of 
these  "  stuffs,"  we  have  been  made  acquainted 
with  their  less  familiar  components,  such  as 
the  gases  oxygen,  hydrogen,  and  nitrogen, 
which  do  not  come  within  the  popular  notions 
of  matter  unless  attention  is  specially  directed 
91 


92  CHEMISTRY 

to  their  material  nature.  It  is  obvious  that 
many  of  the  substances  referred  to  are 
compounds  made  up  of  at  least  two  kinds  of 
matter,  such,  for  example,  as  iron  rust  and 
iron  scale  composed  of  iron  and  oxygen,  iron 
sulphide  made  up  of  iron  and  sulphur,  or 
water  composed  of  hydrogen  and  oxygen. 

It  thus  falls  within  the  province  of  Chemistry, 
which  is  concerned  with  the  transformations 
of  matter  in  the  sense  which  has  now  been 
made  clear,  to  determine,  in  the  first  place, 
whether  a  substance  which  is  not  a  mixture 
is  composite — whether  other  forms  of  matter 
can  be  got  out  of  it  by  chemical  or  physical 
processes.  It  may  be  well  to  take  advantage 
of  this  opportunity  for  pointing  out  that  to 
the  chemist  the  various  forms  of  energy,  heat, 
light,  electricity,  etc.,  are  as  much  chemical 
agencies  as  matter  itself,  in  so  far  as  these 
forms  of  energy  are  capable  of  producing 
chemical  change.  We  investigate  the  com- 
position of  matter  both  by  physical  processes 
and  by  purely  chemical  processes,  meaning 
by  the  latter  the  action  of  one  form  of  matter 
upon  another.  This  last  case  is  also  ulti- 
mately resolvable  into  terms  of  energy  ;  but 
for  the  present  the  distinction  will  be  found 
convenient.  The  study  of  the  composition  of 
matter  can  be  conducted  in  two  ways — 


ELEMENTS  AND  COMPOUNDS  93 

a  substance  may  be  resolved  by  physical  or 
chemical  means  into  its  components,  or  the 
components  may  be  brought  into  combination, 
and  the  substance  built  up.  The  first  process 
is,  in  a  general  way,  described  as  analysis,  and 
the  complementary  process  as  synthesis.  Very 
often  the  composition  of  some  particular  stuff 
te  proved  in  both  ways.  Thus,  when  water 
is  shown  by  the  action  of  hot  iron  to 
contain  oxygen  and  hydrogen,  its  composition 
is  proved  by  analysis  ;  when  water  is  formed 
by  the  combination  of  oxygen  and  hydrogen, 
its  composition  is  proved  by  synthesis.  The 
more  complex  the  substance — the  greater  the 
number  of  different  forms  of  matter  which 
enter  into  its  composition — the  more  important 
and,  it  may  be  added,  the  more  difficult  does 
the  proof  of  its  composition  by  both  methods 
become.  In  the  case  of  extremely  complex 
substances,  it  is  often  not  possible  in  the 
present  state  of  knowledge  to  complete  the 
synthetical  evidence  ;  but,  as  we  are  approach- 
ing the  subject  gradually,  it  will  be  advisable 
at  this  stage  to  concentrate  attention  upon 
the  simpler  cases. 

For  at  least  a  century  and  a  half,  chemists 
have  thus  been  trying  by  every  possible 
method  to  pull  all  available  materials  to  pieces. 
The  component  parts  which  the  chemist  has 


94  CHEMISTRY 

striven  for  are  not  simply  proximate,  but 
ultimate — as  ultimate  as  his  methods  can 
carry  him.  The  mineralogist,  for  instance,  by 
microscopic  examination  or  by  other  methods, 
can  determine  the  proximate  components  of 
a  mineral — he  may  resolve  granite  into  quartz, 
felspar,  and  mica ;  the  biologist  may  dissect 
an  animal  or  a  plant  into  various  organs  and 
tissues,  and,  by  pushing  his  studies  still  further 
into  the  domain  of  cytology,  he  may  study 
the  individual  cells  of  which  all  organisms  are 
composed,  and,  by  the  application  of  the 
highest  magnifying  power  and  the  use  of 
dyes  which  stain  the  different  component 
materials  of  the  cell  selectively,  he  may  effect 
a  microscopic  analysis  of  even  the  minutest 
organized  units  of  animals  and  plants.  Or 
the  mechanic  may  separate  a  machine  into 
its  component  parts  to  find  out  how  it  works, 
and  he  may  put  the  parts  together  again  and 
restore  the  mechanism  to  its  working  condi- 
tion ;  his  analysis  need  go  no  further  for  his 
purpose  than  the  separation  of  wheels,  cranks, 
levers,  and  so  forth.  But  the  chemist  pushes 
his  inquiries  deeper  than  this  ;  he  can  tell  the 
mineralogist  what  the  components  of  granite 
are  themselves  composed  of,  he  can  supply 
the  biologist  with  information  concerning  the 
different  complex  forms  of  matter  of  which 


ELEMENTS  AND  COMPOUNDS  95 

the  organized  units  are  composed,  and  he  can 
resolve  the  component  parts  of  a  machine 
into  a  few  metals,  upon  the  specific  properties 
of  which  the  efficiency  of  the  machine  depends. 

The  general  result  of  the  study  of  matter 
from  this  chemical  point  of  view  has  been 
the  discovery  that  the  process  of  resolution 
in  every  case  reaches  a  limit.  From  whatever 
materials  we  set  out,  there  are  finally  obtained 
some  forms  of  matter  which  cannot  be  further 
decomposed — from  which  nothing  different 
can  be  produced  by  any  known  process  of 
resolution  ;  substances  whch  are  said  to  be 
elementary.  So  that  from  the  chemist's 
standpoint  all  matter  is  either  resolvable  or 
unresolved ;  the  Universe  is  built  up  of 
elementary  and  compound  matter.  Whether 
elementary  matter  is  in  its  ultimate  nature 
unresolvable  is  a  question  for  the  future  to 
decide  ;  the  point  is  raised  in  connection  with 
recent  researches  in  Radioactivity.  But  for 
all  practical  purposes  it  may  be  assumed  that, 
whether  further  decomposition  of  elementary 
matter  is  possible  or  not,  no  such  resolution 
takes  place  in  the  course  of  any  ordinary  case 
of  chemical  change. 

The  Chemical  Elements. — The  component 
parts  of  anything  may,  therefore,  be  said  to 
be  its  elements,  the  degree  of  resolution 


96  CHEMISTRY 

depending  very  much  upon  the  particular 
branch  of  science.  Thus,  to  the  anatomist, 
the  limbs,  organs,  skeleton,  etc.,  are  the 
elements  of  an  animal ;  the  bones  are  the 
elements  of  the  skeleton ;  the  histologist 
separates  tissues  into  elementary  cells  ;  and 
to  the  cytologist  the  various  components  of 
the  cell  constitute  its  elements.  Or,  to  the 
engineer,  the  wheels,  cranks,  etc.,  of  a  machine 
are  its  elements.  The  separation  of  matter 
by  chemical  methods  is  evidently  more 
fundamental ;  and  the  chemical  elements  are 
recognized  in  every  department  of  science. 
The  evolution  of  the  idea  of  an  element  in 
Chemistry  forms  an  interesting  chapter  in  the 
history  of  the  science,  and  the  student  will 
find  the  story  most  fascinating.  It  will  be 
readily  understood  that,  with  the  discovery 
of  new  methods  of  decomposing  matter  chemi- 
cally, substances  apparently  elementary  were 
resolved.  In  this  way,  the  list  of  elements 
was  from  time  to  time  revised  by  the  substitu- 
tion of  new  elements  for  their  previously 
unresolved  compounds.  Then,  again,  with 
increasing  activity  in  the  study  of  rare 
minerals  and  out-of-the-way  materials — with 
refinement  of  methods  of  separation,  and 
with  improvements  in  physical  and  chemical 
methods  of  detection  and  discrimination — 


CHEMICAL   ELEMENTS  97 

the  number  of  elements  has  been  increased 
until  at  the  present  time  about  eighty — 
according  to  the  latest  census  eighty-two — 
of  these  unresolved  "  stuffs "  are  known. 
When  the  reader  hears  of  "  new  "  chemical 
elements  being  discovered,  he  will,  of  course, 
understand  that  what  is  really  meant  is  the 
detection  of  some  element  that  had  previously 
escaped  notice  by  virtue  of  its  rarity,  or  on 
account  of  its  being  difficult  to  separate  from 
associated  matter,  or  because  of  its  lacking 
obtrusively  distinct  characters.  No  chemist 
has  ever  done  more  than  bring  to  light  those 
raw  materials  of  the  Universe  which  were 
already  in  existence  ages  before  the  advent  of 
man  upon  this  earth. 

It  is  the  business  of  the  chemist  to  know 
as  much  as  possible  about  these  elements  ; 
to  acquaint  himself  with  their  mode  of  occur- 
rence in  nature,  with  the  methods  of  isolating 
them,  and  with  their  characteristic  properties 
as  individual  forms  of  matter — in  short,  with 
their  natural  history.  The  student  need  not 
be  appalled  at  the  magnitude  of  the  field 
thus  opened  out ;  he  will  not  be  called  upon 
to  commit  to  memory  long  tables  of  facts  and 
figures — all  has  been  systematized  and  simpli- 
fied by  scientific  generalization.  The  chemical 
elements  play  very  different  parts  in  the 


98  CHEMISTRY 

economy  of  nature,  and  in  their  utility  in  the 
arts  and  manufactures.  Some  are  abund- 
antly distributed  both  in  the  free  and  combined 
state ;  others  are  also  abundant,  but  occur 
only  in  combination ;  others  are  rare  in  the 
free  state,  while  their  compounds  are  common  ; 
and  others,  again,  are  extremely  scarce  in  any 
state.  It  is  not  proposed,  even  if  it  were 
possible,  in  this  little  volume  to  set  forth  more 
than  a  few  generalities  concerning  the  eighty- 
two  elements  known  to  science,  since  any 
invidious  selection  would  necessarily  convey 
a  false  impression  of  relative  importance.  In 
applied  chemistry,  in  the  economy  of  life, 
in  their  capability  of  forming  multitudinous 
compounds,  every  set  of  elements  has  its  order 
of  importance ;  in  the  light  of  pure  science, 
there  is  no  absolute  scale  of  importance ; 
every  element  has  its  own  story  to  tell,  and 
one  which  occurs  only  in  infinitesimal  traces — 
the  newly  isolated  radium — has  opened  up 
some  of  the  most  fundamental  questions 
concerning  the  ultimate  constitution  of  matter 
that  have  ever  been  raised  since  the  individ- 
uality of  the  elements  became  a  recognized 
scientific  doctrine. 

Acquaintance  with  some  of  the  chemical 
elements  has  already  been  made  in  the  pre- 
ceding chapters ;  and  it  will  have  been 


CHEMICAL   ELEMENTS  99 

realized  that  the  conception  of  an  element  as 
an  unresolved  form  of  matter  is  quite  inde- 
pendent of  the  physical  state  of  aggregation. 
Thus,  oxygen,  hydrogen  and  nitrogen  are 
gaseous  elements  under  ordinary  conditions  ; 
sulphur  and  iodine  are  solids.  Iron,  copper, 
zinc  and  magnesium  are  all  elements,  and 
have  been  mentioned  as  belonging  to  the 
category  of  metals — they  are  metallic  elements, 
and  are  all  solid  at  ordinary  temperatures, 
but  have  definite  points  of  liquefaction,  i.e., 
melting-points.  The  familiar  metal  mercury, 
or  quicksilver,  is  an  element  that  is  liquid 
under  ordinary  conditions,  but  it  solidifies 
at—  39-5°C.,  and  is  a  colourless  vapour  above 
357°.  It  would  be  possible  to  make  a  classifi- 
cation of  the  elements  based  on  their  physical 
state  as  familiarly  known  to  us  ;  but  nothing 
would  be  gained  by  such  a  classification 
beyond  mnemonical  assistance.  Chemical  clas- 
sification naturally  goes  deeper,  and  is  directed 
towards  the  establishment  of  chemical  rela- 
tionships— the  association  of  elements  which 
are  allied  in  their  chemical  characters.  Solid 
sulphur  and  gaseous  oxygen,  for  example, 
have  many  chemical  properties  in  common. 
No  classification  of  the  elements  based  on 
any  set  of  purely  physical  characters  is  of  any 
use  from  the  chemical  point  of  view,  although, 


100  CHEMISTRY 

as  will  be  seen  subsequently,  the  chemical 
classification  is  of  the  greatest  value  from  the 
physical  point  of  view. 

The  reader  is  invited  to  consider  another 
example  which  will  serve  to  introduce  a  set 
of  four  closely  related  elements,  one  of  which, 
iodine,  has  already  been  made  use  of  in  illus- 
tration of  chemical  change  (p.  38).  Of  these 
four  elements,  the  most  abundant  is  chlorine, 
which  is  a  component  of  common  salt ;  and, 
as  common  salt  is  contained  in  and  was 
originally  obtained  from  sea  water,  the  group 
has  received  the  name  of  Halogens,  which 
simply  means  generators  of  salt  like  sea-salt. 
The  halogens,  then,  are  fluorine,  chlorine, 
bromine  and  iodine.  Fluorine  occurs  in 
combination  as  a  constituent  of  the  minerals 
fluorite  (fluor  spar)  and  cryolite,  the  other 
constituents  of  these  minerals  being  certain 
metallic  elements  which  may  stand  over  for 
further  consideration.  Chlorine  occurs  in 
combination  in  common  salt,  vast  deposits  of 
which  are  found  "  bedded  "  in  certain  geo- 
logical strata  ;  this  salt  is  also  the  main  com- 
ponent of  the  solid  residue  left  by  the  drying 
down  (evaporation)  of  sea  water.  Bromine 
also  is  found  in  combination  as  a  salt  in  the 
same  geological  deposits  with  common  salt ; 
and  iodine  is  found  in  combination  in  the 


CHEMICAL   ELEMENTS  101 

earthy  residue  or  ash  left  when  sea-weeds  are 
burnt.  There  are  also  found  in  Chili  deposits 
of  a  certain  salt  known  as  Chili  Saltpeter, 
which  is  largely  used  as  a  fertilizer  for  crops  ; 
with  this  salt  certain  compounds  of  iodine  are 
found  in  admixture.  Thus,  the  halogens  are 
found  in  nature  only  in  a  state  of  combina- 
tion ;  they  have  been  "  discovered,"  i.e., 
isolated,  by  chemical  methods,  and  their 
elementary  character  has  been  established  by 
chemical  research.  The  question  now  arises — 
why  are  these  four  elements  grouped  together  ? 
That  the  classification  is  independent  of 
physical  considerations  is  shown  by  the 
fact  that  fluorine  and  chlorine  are  gases  at 
ordinary  temperatures,  bromine  a  deep  red 
heavy  liquid,  and  iodine  a  metallic  looking 
solid.  The  relationship  between  the  members 
of  this  family  is  primarily  chemical ;  with 
the  same  element  they  form  compounds  of 
absolutely  similar  types  and  characters.  To 
simplify  matters  at  this  stage,  it  may  be 
considered  that  the  element  which  is  most 
commonly  found  in  combination  with  the 
halogens  under  natural  conditions  is  the 
metallic  element  known  as  sodium.  Common 
salt  is  a  compound  of  chlorine  and  sodium  ; 
and,  just  in  the  same  way  that  we  speak  of 
compounds  of  oxygen  as  oxides  (p.  71),  we 


102  CHEMISTRY 

say  that  salt  is  sodium  chloride.  The  other 
halogens  form  similar  compounds — sodium 
fluoride,  bromide  and  iodide  respectively.  It 
may  be  mentioned  incidentally  that  fluorite 
is  a  compound  of  fluorine  with  another  metallic 
element,  calcium,  of  which  quicklime  (p.  39), 
is  an  oxide ;  while  cryolite  contains,  in  addi- 
tion to  the  fluoride  of  sodium,  the  fluoride  of 
the  metallic  element  aluminium,  of  which 
element  the  mineral  substance  clay  is  one  of 
the  most  familiar  compounds.  Now,  all  the 
sodium  compounds  of  the  halogens  are  alike 
in  their  general  properties ;  they  are  all 
colourless  salts,  crystallizing  in  the  same 
cubical  form,  and  having  the  same  chemical 
characters.  What  is  true  for  sodium  is  true 
for  calcium  and  for  other  elements,  metallic 
or  non-metallic,  so  far  as  concerns  the  simi- 
larity of  the  four  compounds  in  each  set. 
Thus  the  four  halogens  form  a  series  of  four 
compounds  with  hydrogen,  all  colourless 
gases  at  ordinary  temperatures,  and  all  having 
similar  chemical  properties ;  the  fluoride, 
chloride,  bromide  and  iodine  of  hydrogen  are 
as  comparable  among  themselves  as  are  the 
sodium  compounds. 

Thus,  in  the  halogens  we  have  a  natural 
group  or  family  of  elements  which  are  asso- 
ciated together  because  of  their  chemical 


CHEMICAL   ELEMENTS  103 

relationship.  The  profound  significance  of 
chemical  relationship  will  be  more  and  more 
realized  as  the  student  becomes  more  and 
more  familiar  with  the  natural  history  of  the 
individual  elements.  All  the  elements  can  be 
grouped  into  families,  the  members  of  which 
possess  among  themselves  certain  characters 
in  common.  Such  classification  is  the  first 
step  towards  the  systematic  study  of  the 
elements  as  a  whole.  It  is  unnecessary  now 
to  adduce  other  illustrations  ;  but  this  same 
group  will  enable  us  to  lay  hold  of  another 
principle  of  great  importance — the  principle 
of  gradation  of  character.  To  the  chemist, 
this  means  gradation  of  chemical  character. 
We  can,  for  instance,  arrange  the  halogens 
in  a  series  in  the  order  of  their  chemical 
activity.  Some  notion  of  what  is  meant  by 
chemical  activity  has  already  been  given 
(p.  83).  The  order  from  this  point  of  view  is 
(1)  fluorine,  (2)  chlorine,  (3)  bromine,  (4)  iodine. 
Fluorine  is  the  most  energetic,  iodine  the  least ; 
fluorine  combines  so  energetically  with  other 
elements,  and  forms  such  stable  compounds, 
that  it  is  one  of  the  most  difficult  elements  to 
isolate,  the  difficulty  arising  from  the  circum- 
stance that  it  attacks  all  the  ordinary  materials 
of  which  chemical  vessels  are  made ;  and  its 
isolation  was  only  made  possible  by  using 


104  CHEMISTRY 

an  apparatus  constructed  of  the  metal  plat- 
inum, which  is  so  inert  towards  all  the  chemical 
elements  that  it  resists  the  attack  of  the 
intensely  active  element  (Moissan,  1886), 
The  relative  activities  can  be  illustrated  by 
the  statement  that  fluorine  decomposes  chlor- 
ides, bromides  and  iodides,  turning  out  the 
halogen  from  these  compounds  and  forming 
fluorides  ;  chlorine  similarly  displaces  bromine 
and  iodine ;  and  bromine  displaces  iodine. 

From  considerations  of  this  sort  we  are  thus 
enabled  to  form  the  graduated  series  given 
above.  Having  formed  this  series  on  chemical 
grounds,  then,  physical  properties  reveal  them- 
selves as  also  gradational.  Take  the  boiling- 
points  for  example :  fluorine,  —  187°;  chlorine, 
-  35°  ;  bromine,  59°  ;  iodine,  184°.  Or  the 
melting-points :  fluorine,  about  —  223° ;  chlor- 
ine, --  102°;  bromine,  -  7-3°;  iodine,  114°. 
Or  the  colour  :  fluorine,  pale  greenish  yellow  ; 
chlorine,  deeper  greenish  yellow ;  bromine, 
deep  red  ;  iodine,  violet  vapour.  From  such 
facts  as  these — and  other  chemical  families 
also  show  gradation  of  characters — it  follows 
that  chemical  relationships  are  expressive  of 
some  deep-seated  properties  inherent  in  the 
ultimate  constitution  of  matter.  What  these 
ultimate  properties  are  may  be  brought  to 
light  by  future  research  ;  in  the  present  state 


CHEMICAL    ELEMENTS  105 

of  knowledge  we  simply  deal  with  them  as 
they  are  presented  to  us.  The  gradation, 
as  will  be  seen  later,  is  also  associated  with 
another  fundamental  attribute,  mass,  as 
measured  by  weight ;  but  this  belongs  to 
another  chapter  in  which  the  classification  of 
the  whole  body  of  chemical  elements  has  to  be 
dealt  with. 

It  will  be  now  seen  that  there  is  justifi- 
cation for  the  statement  made  above  that 
the  chemical  classification  of  the  elements 
is  of  importance  in  Physics,  because  in  a 
graduated  series  the  physical  properties  can 
be  inferred  within  certain  broad  limits  from 
the  position  of  the  element  in  the  series.  Thus, 
knowing  the  boiling-points  of  fluorine  and 
bromine,  it  could  be  predicted  that  the  boiling- 
point  of  chlorine  would  lie  somewhere  between, 
or,  knowing  the  descending  order  of  boiling- 
points  from  iodine  to  chlorine,  it  could  have 
been  foreseen  that  the  boiling-point  of  fluorine, 
when  determined,  would  be  found  to  be  the 
lowest  of  the  series.  Still  more  potent  as  a 
scientific  weapon  would  be  any  larger  scheme 
of  classification  which  comprised  all  the 
smaller  natural  groups  of  elements,  and  enabled 
the  prevision  of  properties  to  be  made  with  a 
greater  degree  of  accuracy — with  a  precision 
measured  by  narrow  instead  of  by  broad 


106  CHEMISTRY 

limits.     That  such  a  scheme  exists  will  be 
made  clear  in  a  later  chapter. 

Metals  and  Non-Metals. — In  the  course  of 
the  preceding  section,  a  general  grouping  of 
the  elements  has  tacitly,  if  not  explicitly,  been 
adopted.  Several  elements,  such  as  iron, 
copper,  zinc,  magnesium,  sodium,  calcium, 
and  aluminium,  have  been  spoken  of  as 
metals.  It  is  customary  to  speak  of  the 
elements  as  metallic  or  non-metallic — as 
metals  or  non-metals.  Here  again,  we  have 
a  classification  in  which  chemical  properties 
are  expressive  of  some  correlated  physical  dis- 
tinction between  the  groups.  It  is  not  possible 
to  draw  a  hard  and  fast  line  between  the  two 
divisions  ;  but  in  a  general  way,  and  as  the 
result  of  even  the  most  casual  observation, 
the  reader  cannot  have  failed  to  have  asso- 
ciated certain  characters  with  the  term  metal. 
Of  course,  many  familiar  metals  are  not  ele- 
mentary, but  mixtures — so-called  alloys — or, 
to  speak  more  accurately,  either  simple 
mixtures  or  solid  solutions  of  two  or  more 
elementary  metals.  Brass,  for  instance,  is 
an  alloy  of  copper  and  zinc ;  bronze,  gun- 
metal,  and  bell-metal  are  alloys  containing 
the  elements  copper  and  tin,  etc.  ;  the 
"  silver  "  of  the  coinage  is  not  the  element 
silver,  but  an  alloy  of  silver  and  copper ; 


METALS   AND    NON-METALS     107 

the  "  gold  "  of  our  coinage  is  an  alloy  con- 
taining 2  parts  of  copper  in  24  parts  of  gold. 
Pewter  and  soft  solder  are  alloys  containing 
the  elements  tin  and  lead ;  steel,  one  of  the 
materials  most  widely  used  in  constructive 
engineering  work,  is  the  metallic  element 
iron  containing  a  small  percentage  of  carbon. 
The  physical  properties  of  the  metals  are 
profoundly  modified  by  this  physical  associa- 
tion with  other  metallic  or  non-metallic 
elements,  so  that  many  of  these  alloys  are  of 
enormous  industrial  importance  for  purposes 
for  which  the  pure  metals  would  be  useless. 
We  are  not  specially  concerned  here  with  this 
branch  of  applied  chemistry — it  belongs  to  a 
large  and  important  subject  which  is  studied 
under  the  designations  Metallurgy  and  Metal- 
lography, and  special  works  must  be  consulted 
for  detailed  information. 

From  the  chemical  point  of  view,  one  of  the 
chief  distinctions  between  the  two  groups  is 
to  be  found  in  the  nature  of  the  compounds 
which  they  form  with  oxygen — their  oxides 
(p.  71).  Of  the  elements  already  referred  to, 
hydrogen,  nitrogen,  carbon,  sulphur,  and  the 
halogens  are  non-metals.  Now,  these  elements 
can  all  be  made  to  combine  with  oxygen  (which 
is  also  a  non-metal) — sometimes  directly, 
as  in  the  case  of  hydrogen,  sulphur,  carbon 


108  CHEMISTRY 

and  nitrogen,  and  sometimes  indirectly,  as 
in  the  case  of  the  halogens.  All  these  oxides 
of  the  non-metals,  however  formed,  are,  with 
the  exception  of  water,  which  is  a  neutral 
compound,  possessed  of  certain  properties 
which  are  described  as  acid.  Their  solutions 
in  water  are  sour  to  the  taste,  and  redden 
the  blue  vegetable  colouring-matter  litmus. 
These  acids,  moreover,  have  the  property 
of  combining  with  the  metallic  oxides  by  which 
the  said  acids  are  more  or  less  neutralized, 
giving  rise  to  the  formation  of  those  extremely 
important  compounds  known  by  the  general 
name  of  salts,  about  which  more  remains  to 
be  said.  The  oxides  of  the  metals  are  known 
as  bases  (in  contradistinction  to  acids) ;  so 
that  the  general  distinction  between  the 
groups  may  be  summarized  by  the  statement 
that  the  non-metals  form  acid  oxides  which 
in  aqueous  solution  are  sour  and  redden 
litmus,  while  the  metals  form  basic  oxides 
which  neutralize  the  acids  to  form  salts  and 
which  restore  the  blue  colour  of  litmus.  Of 
course,  the  distinction  is  not  absolute — there 
are  some  acid  metallic  oxides,  but  there  are 
no  basic  non-metallic  oxides.  Neither  must 
it  be  inferred  that  the  property  of  conferring 
acidity  exclusively  pertains  to  oxygen,  since 
the  compounds  of  the  halogens  with  hydrogen 


METALS    AND    NON-METALS     109 

(p.  102)  are  also  acids,  although  they  contain  no 
oxygen.  But  in  general  terms  the  distinction 
is  sound ;  it  corresponds  with  the  facts,  and 
is  further  borne  out  by  another  distinctive 
character  which  will  introduce  a  new  set  of 
considerations. 

Beginning,  as  before,  with  a  simple  case, 
common  salt  may  be  taken  as  a  type  of 
the  class  of  compounds  termed  salts.  It  has 
already  been  stated  that  this  substance  is  a 
compound  of  the  metal  sodium  with  the  non- 
metal  chlorine.  Although  metallic  and  non- 
metallic  oxides  combine  as  just  stated  to  form 
salts,  sodium  chloride  is  also  a  salt  in  the 
chemical  sense,  so  that  there  may  be  salts 
with  or  without  oxygen — haloid-salts  and 
oxy-salts.  The  various  methods  by  which 
salts  can  be  formed  do  not  now  enter  into 
consideration ;  they  can  be  produced  by 
other  means  than  the  combination  of  acids 
and  bases,  but,  however  formed,  they  are 
capable  of  being  decomposed  by  electricity. 
A  salt,  either  in  solution  in  water  or  in  a  state 
of  fusion  by  heat — if  it  can  be  fused — is 
resolved  by  a  current  of  electricity ;  it  is 
said  to  be  an  electrolyte,  and  to  undergo 
electrolysis.  The  same  is  true,  it  may  be  said 
parenthetically,  of  acids  and  bases ;  these 
also  are  electrolytes.  Now,  electrolysis  is  a 


110  CHEMISTRY 

process  of  chemical  decomposition  by  physical 
agency  (p.  92) ;  and  the  products  of  electrolysis 
are  the  original  components  of  the  salt,  acid, 
or  base,  or  the  secondary  products  of  the 
chemical  change  resulting  from  the  interaction 
of  the  primary  products  and  the  solvent. 
Thus,  if  one  of  the  products  of  electrolysis  is 
a  metal  which  decomposes  water  at  ordinary 
temperatures,  it  is  evident  that  we  should  not 
get  that  metal  by  electrolysing  an  aqueous 
solution  of  one  of  its  salts,  but  the  products 
of  the  interaction  of  the  metal  and  water, 
one  such  product  being  hydrogen  (p.  83). 
Sodium  is  one  of  the  metals  which  decompose 
water  at  ordinary  temperatures,  so  that,  if 
a  current  of  electricity  is  passed  through  an 
aqueous  solution  of  common  salt,  the  products 
are  chlorine  and  hydrogen — not  chlorine  and 
sodium.  If  dry  fused  sodium  chloride  is 
electrolysed,  the  products  are  sodium  and 
chlorine. 

The  general  result  is  that  the  primary 
products  may  always  be  regarded  as  a  metal 
and  the  other  component  of  the  salt,  whatever 
that  may  be.  If  we  are  dealing  with  a  solution 
of  an  oxy-salt,  the  products  are  hydrogen 
and  oxygen.  But  the  oxygen  is  in  this  case 
a  secondary  product,  since  the  salt  is  resolved 
into  metal  (or  hydrogen,  if  the  latter  decom- 


METALS    AND    NON-METALS     111 

poses  water),  and  the  whole  oxygen-containing 
group,  which  latter  group  decomposes  water 
with  the  liberation  of  oxygen.  The  oxygen- 
containing  group  is  the  electro-negative 
equivalent  of  the  halogen  in  sodium  chloride. 
The  details  of  electrolytic  decomposition  and 
their  theoretical  explanation  are  dealt  with 
in  works  on  electro-chemistry.  The  main 
fact  which  this  introduction  is  intended  to 
bring  out  is  that  the  metal  and  the  halogen 
or  oxygen-containing  group  always  travel  in 
opposite  directions — the  one  component  of 
the  salt  appears  at  one  pole  or  electrode,  and 
the  other  component  (or  the  secondary  pro- 
ducts) at  the  other  electrode.  And,  in  accord- 
ance with  the  principles  of  electrical  science, 
the  metal,  which  always  appears  at  the 
negative  electrode  (or  kathode)  is  said  to  be 
electro-positive,  and  the  halogen  or  other 
acid  group,  which  is  liberated  at  the  positive 
electrode  (or  anode),  is  said  to  be  electro- 
negative. So  that  there  is  a  physical  classifi- 
cation of  the  elements  thus  made  possible 
according  to  the  behaviour  of  their  compounds 
on  electrolysis ;  and  in  broad  terms  this 
classification  agrees  with  the  division  into 
metals  and  non-metals,  the  former  being  as  a 
group  electro-positive,  and  the  latter  electro- 
negative. It  must  be  noted  that  the  non- 


112  CHEMISTRY 

metal  hydrogen  is  an  exception,  since  it  is 
distinctly  electro-positive.  But  this  excep- 
tion has  a  special  significance,  because,  in 
most  of  its  chemical  relationships,  hydrogen 
is  more  closely  allied  to  the  metals  than  is 
any  of  the  other  non-metallic  elements  ;  and 
it  would  not  be  straining  matters  unduly  even 
now  to  class  hydrogen  with  the  metals  if 
only  its  chemical  characters  are  taken  into 
consideration. 

Out  of  the  whole  list  of  elements,  eighteen 
may  be  labelled  non-metals.  Here,  again,  there 
is  an  indication  that  chemical  character  is 
expressive  of  something  profound  and  inher- 
ently constitutional  in  matter,  because,  as 
groups,  the  metals  and  the  non-metals  differ 
in  many  well-known  characters.  Thus,  the 
metals  are  opaque  (excepting  when  in  very 
thin  films)  and,  when  in  a  massive  state,  as 
distinguished  from  a  pulverulent  condition, 
possessed  of  "  lustre " — a  property  well 
exemplified  by  the  familiar  appearance  of 
polished  silver,  gold,  or  copper.  Among  the 
non-metals,  iodine  is  the  most  pronounced 
exception,  as  its  crystals  are  possessed  of 
distinctly  metallic  lustre.  In  contradistinc- 
tion to  the  opacity  of  the  metals,  ten  of  the 
non-metals  are  transparent  gases  at  ordinary 
temperatures.  The  metals  as  a  class  are, 


METALS    AND    NON-METALS     113 

in  varying  degrees,  conductors  of  heat  and 
of  electricity,  while  the  non-metals  are  bad 
conductors  or  non-conductors.  Among  the 
solid  non-metallic  elements,  such  metallic 
properties  as  malleability  or  tenacity  are  not 
met  with. 

With  respect  to  the  distribution  of  the  ele- 
ments, it  is  of  interest  to  note  that  more  than 
three-fourths  of  the  accessible  crust  of  this 
globe  upon  which  we  live  is  made  up  of  the  two 
non-metals,  oxygen  and  silicon,  about  one-half 
being  oxygen.  Nothing  affords  more  striking 
evidence  of  the  marvel  of  chemical  change 
than  the  contemplation  of  this  geo-chemical 
fact,  that  the  superficial  "  solidity  "  of  the 
earth  is  due  to  the  predominance  of  those 
mineral  constituents  into  the  composition  of 
which  gaseous  oxygen  and  the  non-metal 
silicon  enter  to  a  preponderating  extent. 
The  whole  crust  of  the  earth  with  which 
geology  deals  is  composed  to  the  extent  of 
more  than  99  per  cent,  of  only  about  twenty 
out  of  the  eighty-two  elements.  This  will  give 
an  idea  of  the  rarity  of  some  of  the  materials 
which  the  chemist  has  had  to  deal  with. 
Out  of  the  twenty  elements  which  predominate 
in  the  earth's  crust,  four  non-metals  and  seven 
metals  exist  in  quantities  which  have  been 
estimated  to  constitute  more  than  99  per  cent. 


114  CHEMISTRY 

of  the  whole  crust  in  the  proportions  by  weight 
given  below  (F.  W.  Clarke) : — 

Oxygen,*  49-98 ;  Silicon,*  25-3 ;  Alum- 
inium^ 7.26;  Iron,f  5-08;  Calcium,f  3-51; 
Magnesium,")*  2-5  ;  Sodium, "j-  2-28  ;  Potas- 
sium^ 2-23;  Hydrogen,*  0-94;  Titanium,  f 
0-3  ;  Carbon,*  0-21. 

The  actual  mode  of  combination  of  the 
chemical  elements  as  found  in  nature,  i.e., 
the  chemical  composition  of  the  materials 
composing  the  crust  of  the  globe,  is  dealt 
with  in  works  on  Mineralogy. 

*  Non-metals.  f  Motala. 


CHAPTER   V 

CHEMICAL  EQUIVALENCE — ELECTRO-CHEMICAL 
EQUIVALENCE MULTIPLE  AND  RECIPRO- 
CAL EQUIVALENCE THE  ATOMIC  THEORY 

Chemical  Equivalence. — From  the  principles 
of  the  definiteness  of  chemical  change  (p.  69) 
and  the  Conservation  of  Mass  (p.  76),  it  is 
but  a  short  step  to  the  principle  of  chemical 
equivalence.  As  with  all  broad  scientific 
generalizations,  the  conception  which  the 
reader  is  now  asked  to  grasp  is  simple  enough 
as  an  abstract  notion,  although  some  difficulty 
may  at  first  be  experienced  in  mastering  it. 
It  is,  perhaps,  needless  to  repeat  that  by 
laboratory  work  the  principle  has  been 
deduced,  and  by  quantitative  manipulation 
only  can  its  significance  be  fully  realized. 
It  is  instructive  as  a  chapter  in  the  history  of 
Chemistry  to  read  how  nearly  some  of  the 
earlier  investigators  approached  without  actu- 
ally reaching  the  principle  of  equivalence. 
The  historical  treatment,  however,  cannot  be 
attempted  here — it  must  suffice  to  ascertain 

115 


116  CHEMISTRY 

the  meaning  of  the  term  as  used  now,  without 
going  over  the  steps  by  which  the  present 
position  has  been  reached.  Sufficient  material 
in  the  way  of  facts  and  illustrations  has  been 
given  to  enable  the  subject  to  be  dealt  with 
in  a  general  way. 

In  the  first  place,  it  must  be  noted  that  the 
terms  "  equivalence  "  or  "  equivalent  "  are 
expressive  of  facts  only,  and  are  independent 
of  any  theory  or  explanation.  The  facts  in 
this  case  are  those  ascertained  experimentally 
and  enunciated  in  the  previous  chapters — the 
idiosyncrasies  of  the  chemical  elements,  by 
virtue  of  which  each  element  has  its  own 
special  faculty  of  entering  into  partnership 
with  other  elements  only  in  certain  fixed 
proportions  by  weight.  Out  of  these  facts, 
there  has,  however,  been  developed  one  of  the 
most  illuminating  theories  that  has  been 
introduced  into  our  science,  so  that  the 
importance  of  equivalence  has  always  to  be 
realized  by  the  student  at  a  very  early 
stage.  In  approaching  this  subject,  it  must 
be  pointed  out  that  we  are  not  dealing 
with  the  likes  and  dislikes  of  the  elements 
— why  the  elements  show  preferences  such, 
for  example,  as  that  of  iron  for  oxygen  while 
this  same  metal  refuses  to  combine  under 
similar  conditions  with  nitrogen  (p.  37).  In 


EQUIVALENCE  117 

considering  the  principle  of  equivalence  we 
have  nothing  to  do  with  the  question  why  in 
particular  cases  chemical  union  is  possible  or 
impossible,  but  only  with  the  quantitative 
relationship  between  what  we  may  call  the 
contracting  parties,  i.e.,  the  elements  and 
compounds  which  can  and  do  enter  into 
combination  directly,  or  which  can  be  made 
to  combine  by  indirect  methods. 

From  the  facts  that  combination  does  take 
place  in  fixed  proportions  by  weight,  and  that 
the  product  of  chemical  union  is  homogeneous 
(Chap.  II.),  it  follows  that  any  such  product, 
or,  in  other  words,  any  particular  chemical 
compound,  however  and  whenever  produced, 
always  consists  of  the  same  elements  com- 
bined in  the  same  proportions  by  weight. 
This  appears  almost  a  truism  now,  because 
it  is  obvious  that,  if  there  were  any  latitude  or 
variability  in  the  ratio  in  which  two  or  more 
elements  combined  to  form  a  compound, 
there  would  be  no  homogeneity — we  should 
have  a  mixture  of  two  or  more  compounds. 
The  principle  is  generally  formulated  as  the 
law  of  Constancy  of  Composition  ;  and  although 
it  may  appear  obvious  now,  it  was  at  one  time 
the  subject  of  much  controversy,  arid  this 
chapter  of  history  is  well  worthy  of  considera- 
tion as  an  illustration  of  the  method  by  which 


118  CHEMISTRY 

troth  is  wrong  from  Nature  by  rigid  experi- 
ment and  logical  reasoning.  From  the  point 
of  view  of  this  law,  it  will  be  recognized  that 
a  chemical  compound  may  be  looked  upon 
as  being  as  true  and  as  definite  an  individual 
form  of  matter  as  a  chemical  element.  And 
it  will  be  further  realized  that,  as  the  com- 
plexity of  chemical  compounds  increases,  it 
may  become  more  and  more  difficult  to  prove 
the  individuality  in  particular  cases.  But 
the  law  is  not  thereby  violated — there  may  be 
apparent  indefiniteness,  but  this  is  the  result 
of  the  imperfection  of  our  practical  methods, 
and  the  modern  chemist  still  has  faith  in  the 
principle. 

For  the  elucidation  of  the  principle  of 
equivalence,  a  few  simple  cases  may  be 
considered.  Oxygen  and  the  halogens  all 
form  compounds  with  hydrogen,  directly 
or  indirectly.  Oxygen  and  hydrogen  explode 
when  ignited  to  form  water;  fluorine  and 
hydrogen  explode  spontaneously  when  mixed  ; 
chlorine  and  hydrogen  explode  when  heated, 
or  on  exposure  to  bright  light ;  and  bromine 
and  iodine  can  also  be  combined  with  hydrogen 
directly  or  indirectly.  We  are  not  now 
concerned  with  methods,  bat  with  products. 
The  proportions  by  weight  of  the  elements 
contained  in  these  compounds  are  known 


EQUIVALENCE  119 

with  great  accuracy,  both  from  analytical  and 
synthetical  evidence.  In  round  numbers, 
and  referring  all  the  weights  to  the  standard 
of  one  part  of  hydrogen,  water  contains  8 
parts  of  oxygen,  hydrogen  fluoride  19  parts  of 
fluorine,  hydrogen  chloride  35-2  parts  of 
chlorine,  hydrogen  bromide  79-4  parts  of 
bromine,  and  hydrogen  iodide  126  parts  of 
iodine.  All  these  different  weights  of  the 
respective  elements,  therefore,  satisfy  the 
same  weight  of  hydrogen — they  are  equivalent, 
or  equal  in  value,  from  the  point  of  view  of 
combining  with  a  unit  weight  of  hydrogen. 
Or  take  the  case  of  those  metals  which 
decompose  water  (p.  83).  If  this  process  is 
followed  quantitatively,  it  is  found  that,  for 
every  unit  weight  of  hydrogen  liberated,  there 
are  used  up  12-2  parts  of  magnesium,  21  parts 
of  iron,  23  parts  of  sodium,  and  32-7  parts 
of  zinc.  Here,  again,  there  is  equivalence— 
these  various  weights  represent  quantities  of 
different  elementary  substances  which  are  of 
the  same  chemical  value  as  measured  by 
their  capacity  for  displacing  the  same  weight 
of  hydrogen.  Moreover,  since  this  same 
weight  of  hydrogen  is  equivalent  to  8  parts  of 
oxygen,  these  equivalent  weights  of  the 
metals  are  also  equivalent  to  8  parts  of 
oxygen.  And,  since  the  compounds  formed 


120  CHEMISTRY 

by  the  action  of  water  upon  these  metals 
under  the  conditions  specified  are  oxides, 
or,  as  in  the  case  of  sodium,  may  be  looked 
upon  as  arising  from  the  combination  of  the 
oxide  with  water,  it  follows  that  the  ratio 
between  the  weights  of  the  metal  and  oxygen 
can  also  be  expressed  numerically  on  the 
oxygen  scale  : — Magnesium  oxide,  12-2  :  8  ; 
iron  oxide  ("  scale  "),  21  :  8  ;  sodium  oxide, 
23  :  8  ;  zinc  oxide,  32-7  :  8.  And  so,  by 
ascertaining  the  proportion  of  metal  to  oxygen 
in  other  oxides,  the  equivalents  of  other 
metals  with  reference  to  oxygen  and  (by 
implication)  to  hydrogen  could  be  found. 

The  doctrine  of  equivalence  is,  therefore, 
nothing  more  than  the  numerical  expression 
of  the  definiteness  of  chemical  change  (Chap. 
III.).  Perhaps  it  would  be  more  correct,  if 
we  were  dealing  with  the  subject  in  historical 
sequence,  to  say  that  the  definiteness  of 
chemical  change  is  the  expression  of  the 
principle  of  equivalence,  since  the  definiteness 
was  established  by  quantitative  studies  of  the 
kind  illustrated  above.  But  the  order  of 
statement  is  immaterial  so  long  as  the  principle 
is  understood.  The  "  equivalent  "  is,  thus, 
in  abstract  terms,  a  number  expressing  the 
parts  by  weight  in  which  an  element  combines 
with  or  displaces  some  other  element ;  and 


EQUIVALENCE  121 

since  hydrogen  has  the  lowest  equivalent, 
it  will  be  convenient  at  this  stage  to  refer 
these  weights  to  the  hydrogen  standard. 
It  is  obvious  that  the  conception  as  thus 
formulated  is  an  idealized  one,  because 
there  are  many  elements  which  neither 
combine  with  nor  displace  hydrogen ;  so 
that  in  such  cases  the  equivalent  can  only  be 
determined  indirectly  by  reference  to  some 
other  element  of  known  equivalence.  It  will 
be  shown,  also,  that  the  conception  is  not 
restricted  to  elements  :  since  compounds  are 
formed  by  the  combination  of  elements  in 
equivalent  weights,  it  follows  that  if  com- 
pounds combine  among  themselves — such,  for 
example,  as  acid  oxides  and  basic  oxides 
(p.  108) — there  must  also  be  equivalence  in 
such  cases. 

Electro-chemical  Equivalence. — An  electric 
current  from  a  chemical  point  of  view,  and 
not  regarded  simply  as  a  stream  of  electricity 
flowing  through  a  metallic  conductor,  is  a 
decomposing  agent  (p.  109).  In  accordance 
with  the  doctrine  of  the  Conservation  of 
Energy,  a  given  quantity  of  electricity  has 
its  equivalent  in  terms  of  chemical  work.  In 
the  case  of  electrolysis,  the  actual  weights  of 
elements  liberated  in  a  given  time  by  a 
measured  quantity  of  electricity  measure  the 


122  CHEMISTRY 

chemical  work  done,  so  that  the  determination 
of  the  weight  of,  let  us  say,  a  metal  deposited 
electrolytically  in  a  given  time  is  also  a 
measure  of  the  electrical  energy  used  up. 
This  is  of  importance  in  Physics  as  furnishing 
a  chemical  method  for  the  measurement  of 
electric  quantity ;  and  the  electro-chemical 
equivalents  of  some  of  the  metals  such,  e.g., 
as  silver  and  copper,  have  from  this  point  of 
view  been  determined  with  extreme  accuracy. 
But  the  absolute  weight  of  an  element  liberated 
in  a  given  time  by  a  measured  current  acquires 
also  a  chemical  significance  when  we  pass 
from  the  absolute  to  the  relative — when 
the  relative  quantities  of  different  elements 
liberated  in  the  same  time  by  the  same 
current  are  considered.  It  is  immaterial 
in  this  case  how  the  electricity  is  made  to  do 
its  work  ;  the  current  may  decompose  a  salt, 
or  an  acid,  or  a  base  in  solution  or  in  the  dry 
fused  state  (p.  109).  Fused  common  salt,  for 
example,  yields  23  parts  of  sodium  and  35-2 
parts  of  chlorine ;  a  solution  in  water  of 
hydrogen  chloride  gives  for  1  part  of  hydrogen 
35-2  parts  of  chlorine  ;  water,  made  to  yield 
hydrogen  and  oxygen  indirectly  by  an  electro- 
lysable  acid  in  solution  (p.  109),  gives  in  round 
numbers  8  parts  of  oxygen  for  1  part  of  hydro- 
gen ;  the  volume  ratio — 2  of  hydrogen :  1  of 


EQUIVALENCE  123 

oxygen — is  revealed  electrolytically,  as  it  is 
proved  by  chemical  synthesis  (p.  88).  It  is 
unnecessary  to  multiply  examples  at  this  stage. 
The  point  of  supreme  importance  brought  out 
is  that  these  relative  weights  of  electrically 
liberated  elements  are  identical  with  the 
chemical  equivalents,  a  discovery  due  to 
Faraday  (1833).  The  establishment  of  this 
law  has  led  to  some  of  the  most  fundamental 
modern  developments  both  of  Physics  and  of 
Chemistry ;  but  its  adequate  discussion  cannot 
be  attempted  within  the  limits  of  this  work. 
Our  consideration  must  be  restricted  to  its 
chemical  significance  in  the  narrow  sense. 

Multiple  and  Reciprocal  Equivalence. — The 
notion  of  equivalence  would  be  a  very 
simple  one  if  it  could  be  stated  in  general 
terms  that  the  numbers  representing  the 
equivalents  of,  let  us  say,  the  elements 
A,  B,  C,  D  were  always  representative  of 
reciprocal  equivalence — that  whenever  any 
pair  of  these  elements  combined  to  form  a 
binary  compound  (i.e.,  a  compound  containing 
two  elements)  the  latter  could  always  be 
formulated  as  AB,  AC,  AD,  BC,  BD,  CD,  etc., 
in  which  the  juxtaposition  of  letters  may 
be  taken  as  indicating  chemical  combination, 
and  the  letters  as  representing  equivalent 
weights.  But  this  statement  would  be  too 


124  CHEMISTRY 

sweeping — it  is  not  generally  true,  but  only 
partially  true.  The  principle  of  reciprocity 
holds  good  for  the  simple  cases  referred  to  in 
the  preceding  sections  ;  but,  when  the  whole 
of  the  facts  are  taken  into  consideration, 
complications  arise  owing  to  the  circumstance 
that  many  elements  can  combine  with  each 
other  in  more  than  one  proportion,  each 
compound  so  formed  being  a  perfectly  definite 
chemical  individual.  One  example  of  this 
has  already  been  adduced  in  the  case  of  iron, 
which  can  form  three  different  oxides  (p.  71). 
In  all  three  oxides,  the  equivalent  may  be 
considered  to  be  the  weight  of  iron  which 
combines  with  one  equivalent,  i.e.,  8  parts  of 
oxygen.  On  this  standard,  the  equivalent  of 
iron  in  "  scale  "  is  21  ;  in  rust  (which  contains 
70  per  cent,  of  iron,  and  therefore  30  per  cent, 
of  oxygen),  the  equivalent  of  iron  is  18 f ; 
and  in  the  other  oxide,  which  contains  77-8 
per  cent,  of  iron  and  22-2  per  cent,  of  oxygen, 
iron  has  the  equivalent  28.  Iron,  therefore, 
in  relation  to  oxygen  has  not  one  equivalent, 
but  three  equivalents. 

Any  number  of  cases  could  be  adduced ; 
and,  since  the  principle  of  multiple  equivalence 
is  of  fundamental  importance,  a  few  other 
examples  may  be  given.  Twenty-three  parts 
of  sodium  displace  1  part  of  hydrogen  from 


EQUIVALENCE  125 

water,  and  combine  with  8  parts  of  oxygen 
(p.  110).  The  equivalent  of  sodium  with  refer- 
ence to  oxygen  ( =  8)  is  in  this  case  23.  But 
when  sodium  is  heated  to  about  300°  in  pure 
dry  air,  it  forms  another  oxide  in  which  11 J 
parts  of  sodium  are  combined  with  8  parts 
of  oxygen.  This  element  has,  therefore, 
with  reference  to  oxygen  two  equivalents, 
11 1  or  23.  The  oxide  of  copper  formed  when 
copper  is  heated  in  oxygen  (p.  84)  contains 
31 1  parts  of  copper  to  8  of  oxygen.  By 
chemical  methods  it  is  possible  to  prepare 
another  oxide  which  contains  63  parts  of 
copper  to  8  of  oxygen  ;  so  that  the  equivalent 
of  copper  may  be  31 1  or  63.  Carbon  burns  in 
oxygen  to  form  that  dioxide  which  was 
referred  to  as  a  constituent  of  the  atmosphere 
(p.  36) ;  this  gaseous  compound  of  carbon 
contains  3  parts  of  carbon  and  8  parts  of 
oxygen.  When  carbon  dioxide  is  heated  in 
contact  with  more  carbon,  another  (gaseous) 
oxide  is  formed  which  contains  6  parts  of 
carbon  to  8  of  oxygen.  Moreover,  when 
carbon  is  intensely  heated  by  an  electric  arc 
in  an  atmosphere  of  hydrogen,  it  combines 
with  that  element  to  form  a  gas — a  compound 
of  carbon  and  hydrogen,  and  therefore  called 
a  hydrocarbon — which  is  known  as  acetylene. 
In  this  gas  1  part  of  hydrogen  is  combined 


126  CHEMISTRY 

with  12  parts  of  carbon,  so  that  this  last 
element  has  the  three  equivalents,  3,  6, 
and  12. 

It  is  clear,  therefore,  that  the  conception 
of  equivalence  is  not  so  simple  as  might  be 
imagined  on  first  approaching  the  subject. 
The  numbers  representing  the  equivalents  are 
not  absolute,  but  relative ;  an  element  A 
may  have  towards  some  other  element  B  a  per- 
fectly fixed  equivalent,  but  towards  another 
element  C,  it  may  have  the  same  equivalent 
that  it  has  with  reference  to  B  and  likewise 
another  or  other  equivalents.  A  may  have, 
also,  a  certain  equivalent  with  reference  to 
B  or  C  under  one  set  of  conditions  of  com- 
bination, and  other  equivalents  under  other 
conditions  of  combination.  All  this  has 
been  established  by  quantitative  analysis  and 
synthesis — by  determining  with  the  greatest 
possible  accuracy  the  relative  weights  of  the 
component  elements  contained  in  chemical 
compounds,  or  the  relative  weights  of  the 
elements  which  enter  into  combination  in 
synthetical  operations.  That  these  numerical 
results  are  expressive  of  some  underlying 
physical  reality  is  shown,  also,  by  the  co- 
incidence of  the  chemical  equivalents  with  the 
electro-chemical  equivalents;  because  when 
an  element  has  two  equivalents,  i.e.,  forms 


EQUIVALENCE  127 

two  distinct  compounds  with  another  element, 
it  has  been  proved  to  possess  two  electro- 
chemical equivalents  in  all  those  cases  where 
the  two  electrolysable  compounds  could  be 
practically  dealt  with. 

But,  although  the  conception  of  equivalence 
is  complicated  by  the  multiple  equivalence  of 
many  of  the  elements,  a  glance  at  the  numbers 
given  in  the  preceding  examples  will  show 
that  there  is  simplicity  in  the  apparent 
complexity.  Thus,  the  equivalent  of  carbon 
is  3,  6,  or  12  ;  of  copper  31  \  or  63  ;  of  sodium 
11  \  or  23,  and  so  forth.  These  numbers  stand 
in  the  relationship  of  simple  multiples,  so  that 
the  notion  of  equivalence  must  be  widened 
by  making  allowance  for  the  ratios  of  the 
combining  weights  being  not  only  A :  B,  but 
also  some  simple  multiple  of  A  or  B,  or  of  both, 
such  as  2  A  :  B,  A  :  2  B,  2  A  :  3  B  and  so  forth. 
This  is  what  is  known  in  Chemistry  as  the 
law  of  Multiple  Proportions ;  and  in  this 
sense  only  is  reciprocity  of  equivalence  or 
combining  weight  recognizable.  Thus,  the 
weights  A  and  B  which  combine  with  a  certain 
weight,  C,  of  a  third  element  need  not  neces- 
sarily be  the  same  weights  as  those  which 
exist  in  the  compound  AB,  but  may  be 
some  simple  multiple  such  as  2  A  with  C,  3  B 
with  C,  etc. 


128  CHEMISTRY 

One  illustration  should  give  a  clear  notion 
of  this  principle  of  Reciprocal  Equivalence. 
In  one  of  the  oxides  of  iron,  the  equivalent 
weights  are  iron  :  oxygen  =  28  :  8.  In  iron 
sulphide  (p.  72),  this  same  weight  of  iron  is 
combined  with  16  parts  of  sulphur.  If  there 
were  general  reciprocity  of  equivalence,  it 
would  be  said,  therefore,  that  8  parts  of 
oxygen  should  combine  with  16  parts  of 
sulphur ;  but,  in  fact,  this  is  not  the  case. 
Sulphur  burns  readily  in  oxygen  to  form  an 
oxide — the  so-called  sulphurous  acid  gas, 
or  sulphur  dioxide — which  is  the  oxide 
containing  the  smallest  proportion  of  oxygen, 
viz.,  8  parts  of  oxygen  to  8  of  sulphur. 
No  lower  oxide  is  known,  so  that  8  parts  of 
oxygen  and  16  of  sulphur  are  not  in  this  case 
equivalent ;  in  order  to  express  the  actual 
facts,  we  have  to  take  two  equivalents,  i.e., 
8x2  parts  of  oxygen.  Moreover,  there  is 
another  oxide  of  sulphur  in  which  16  parts 
of  sulphur  are  combined  with  24  parts,  i.e., 
8x3  parts  of  oxygen.  The  doctrine  of 
equivalence  is,  therefore,  only  in  harmony 
with  the  facts  when  sufficient  elasticity  is 
given  to  the  conception  to  allow  of  the 
inclusion  of  cases  of  multiple  equivalence  and 
of  multiple  reciprocal  equivalence ;  and  the 
main  point  brought  out  for  assimilation 


THE    ATOMIC    THEORY         129 

by  the  reader  is  the  simple  and  integral 
character  of  the  numbers  expressing  this 
multiplicity. 

The  Atomic  Theory. — The  doctrine  of  equi- 
valence, even  in  its  most  elastic  form,  is  still 
nothing  more  than  a  quantitative  expression 
of  the  facts  of  chemical  composition.  Of 
course,  there  must  be  some  underlying  prin- 
ciple— some  explanation  of  this  simplicity 
of  multiplicity.  Such  explanation  was  first 
definitely  formulated  in  1807-08  by  John 
Dalton,  who  not  only  discovered  the  law  of 
Multiple  Proportions,  but  suggested  a  theory, 
the  introduction  of  which  marks  one  of  the 
greatest  epochs  in  the  history  of  Chemistry. 
The  reason  why  combination  takes  place  in 
definite  proportions  by  weight,  and  why, 
when  the  same  element  has  more  than  one 
equivalent,  the  principle  of  integral  multiples 
is  maintained  is,  according  to  Dalton's 
explanation,  because  the  combination  is 
between  the  ultimate  particles  of  which 
elementary  matter  is  composed.  This  is  the 
notion  of  the  discontinuity  or  discreteness  of 
matter  referred  to  in  a  former  chapter  (p.  57), 
and  now  re-introduced  in  more  exact  terms. 
The  "  particles  "  of  which  matter  is  composed 
— whatever  its  state  of  aggregation — are, 
from  Dalton's  point  of  view,  ultimate  in  the 


130  CHEMISTRY 

sense  of  being  indivisible.     For  this  reason 
ho  called  them  atoms. 

The  defmiteness  of  combination  by  weight 
is  attributable  to  the  weights  of  the  atoms — 
those  elements  of  the  elements.  The  atomic 
theory  as  thus  introduced  into  Chemistry  was 
the  revival  of  very  ancient  views  concerning 
the  constitution  of  matter ;  but  these 
older  speculations  had  failed  to  influence  the 
progress  of  science  until  given  a  definite 
quantitative  meaning  by  Dalton.  From  the 
point  of  view  of  this  theory,  one  atom 
having  a  certain  weight  can  combine  with 
another  atom  having  also  a  certain  weight, 
or  one  atom  can  combine  with  two,  three, 
etc.,  other  atoms ;  or  again,  two  atoms 
can  combine  with  three,  three  with  four, 
and  so  forth.  Any  degree  of  complexity 
may  exist ;  but,  the  atom  being  (by  hypo- 
thesis) the  ultimate  unit,  the  passage  from 
one  compound  AB  to  another  containing  the 
same  elements  in  different  proportions  cannot 
take  place  but  by  whole  multiples  ;  fractional 
parts  of  A  or  B  are  inadmissible.  It  will  be 
noted  that  at  this  stage  the  theory  deals  only 
with  one  fundamental  attribute  of  matter, 
mass,  as  measured  by  weight.  But,  instead 
of  dealing  with  this  attribute  in  the  gross  as  a 
general  physical  property  of  matter  irrespec- 


THE    ATOMIC    THEORY          131 

live  of  individuality,  the  conception  of  the 
indivisible  atom  brings  the  sciences  of  Physics 
and  Chemistry  into  an  indissoluble  partner- 
ship, for  the  atom  of  each  element  is,  from 
this  standpoint,  a  particle  of  matter  which  is 
indivisible,  and  which  has  a  certain  fixed 
weight,  which  weight  is  specific  for  each 
kind  of  atom,  and  is,  therefore,  a  physico- 
chemical  attribute. 

The  atomic  theory  as  thus  broadly  outlined 
is,  therefore,  based  entirely  upon  the  observed 
facts  respecting  combination  by  weight.  It 
postulates  homogeneity  among  the  atoms  in 
the  sense  that  the  atoms  of  each  element  are 
assumed  to  be  all  alike — all  cast  in  one  mould, 
all  possessed  of  the  same  weight,  size,  shape, 
and  structure.  That  is  why  our  imaginary 
being  (p.  51),  was  supposed  to  be  able  to 
follow  the  movements  of  the  different  "  par- 
ticles "  in  a  mixture  of  gases.  Of  course, 
the  atoms  must  on  this  view  have  absolute 
weights,  definite  dimensions,  shapes,  etc. 
In  its  extreme  form,  the  theory  has  been 
pushed  so  far  as  to  claim  that  the  atom  is 
the  ultimate  component  of  the  material 
universe — imperishable  and  eternal.  The 
reader  must,  however,  discriminate  between 
ascertained  concordance  of  facts  with  theory 
and  speculative  developments  of  theory, 


132  CHEMISTRY 

however  plausible  in  principle  or  stimulating 
in  prompting  further  research.  For  the 
general  purposes  of  chemical  study,  the  atom 
may  be  regarded  as  indivisible.  From  the 
purely  chemical  point  of  view,  the  absolute 
weight  is  immaterial ;  it  answers  all  practical 
purposes  to  deal  with  the  weights,  as  did 
Dalton,  as  relative.  By  physical  methods, 
it  is  possible  to  ascertain  approximately  the 
absolute  weights  and  dimensions  of  atoms, 
but  this  part  of  the  subject  is  beyond  our 
scope ;  suffice  it  to  say  that  the  chemical 
atom  is  a  particle  of  matter  infinitesimally 
small — of  a  minuteness  of  size  and  weight 
which,  even  if  expressed  numerically,  is 
beyond  human  conception. 

The  discussion  of  the  truth  of  the  atomic 
theory — whether  the  atom  is  a  physical  entity 
— belongs  to  the  domain  of  Philosophy ; 
we,  as  chemists,  have  only  to  deal  with  the 
doctrine  in  so  far  as  it  is  in  accordance  with  all 
the  evidence  which  our  science  can  produce, 
and  with  the  verification  of  deductions  drawn 
from  it.  It  accounts  for  the  definiteness  of 
chemical  combination  by  weight,  and,  as  will 
appear  later,  it  correlates  and  explains  large 
bodies  of  facts  which  have  been  accumulated 
since  its  introduction  by  Dalton.  It  is  the 
"  head  stone  of  the  corner  "  in  theoretical 


THE    ATOMIC    THEORY          133 

Chemistry ;  and  yet,  as  it  stands,  we  all 
realize  that  it  is  incomplete — that  vast 
developments  may  yet  be  looked  for  from 
both  the  physical  and  the  chemical  side.  The 
mysteries  of  chemical  change,  the  absolute 
transformation  resulting  from  chemical  com- 
bination, the  likes  and  dislikes  of  the  elements, 
their  activities  and  inactivities,  yet  await 
explanation  in  terms  of  the  atomic  theory. 
The  conception  of  the  atom  has  simply  shifted 
the  responsibility  of  individual  idiosyncrasies 
from  matter  in  the  gross  to  matter  in  detail 
— from  the  bulk  to  the  ultimate  particle. 

The  possibility  of  the  atom  being  itself 
resolvable  did  not  at  first  enter  into  considera- 
tion ;  it  was  assumed  to  be  the  final  limit  of 
the  divisibility  of  matter.  But  the  early 
crude  notions  concerning  the  nature  of  the 
atom  have  gradually  undergone  development, 
chiefly  as  the  result  of  the  study  of  the  action 
of  electricity  upon  matter  in  the  gaseous  state 
(p.  19)  inaugurated  in  this  country  by  Crookes 
and  J.  J.  Thomson.  Physical  modes  of  deal- 
ing with  matter  undreamt  of  in  the  time 
of  Dalton  have  led  to  the  conclusion  that 
the  atom  is  not  a  structureless  particle,  but 
a  complex,  internally  balanced  mechanism, 
capable  under  certain  conditions  of  being 
broken  down  into  its  smaller  component 


134  CHEMISTRY 

parts ;  and  it  is  accepted  by  many  physicists 
and  chemists  as  a  plausible  hypothesis  that 
the  atom  is  built  up  of  particles  of  electricity 
("  electrons  ").  On  this  view,  the  conception 
of  the  material  nature  of  the  atom  would  have 
to  be  recast  in  terms  of  energy  ;  and  it  is 
for  this  reason  that  the  possibility  of  matter 
coming  into  or  going  out  of  existence  as  the 
result  of  chemical  change  was  considered 
worthy  of  being  re-tested  (p.  79).  So  far  there 
is  no  direct  evidence  in  support  of  this  view 
— the  most  carefully  conducted  experiments 
have  only  served  to  confirm  the  doctrine  of 
the  Conservation  of  Mass. 

The  history  of  the  development  of  the 
atomic  theory  will  furnish  fascinating  reading 
for  a  future  generation — experiments  and 
hypotheses  are  now  being  pushed  forward  with 
a  rapidity  and  with  a  boldness  which  entitle 
the  chemist  to  say  that  the  time  is  not  yet 
ripe  for  attempting  to  complete  the  story. 
But  it  is  along  these  lines — the  study  of  the 
inner  mechanism  of  the  atom — that  Physics 
and  Chemistry  are  moving  towards  their 
point  of  convergence.  No  single  attribute 
assigned  to  the  atom  during  the  early  period 
of  the  theory  has  remained  unchallenged. 
The  idea  of  absolute  uniformity  of  weight 
has  been  called  in  question  ;  and  it  has  been 


THE    ATOMIC    THEORY          135 

suggested  that  the  property  is  an  average  one 
— that  there  may  be  a  certain  range  of  varia- 
tion in  weight  among  the  atoms  of  an  element, 
and  that  the  apparent  constancy  in  weight 
is  the  statistical  result  of  the  practical 
necessity  of  dealing  with  myriads  in  our 
laboratory  operations.  Of  such  variability, 
however,  we  have  at  present  no  experimental 
evidence. 

Of  the  shapes  and  textures  of  the  atoms  we 
know  nothing  with  certainty.  The  questions 
whether  they  are  hard  and  rigid,  or  elastic, 
compressible,  and  deformable  have  been 
discussed  from  time  to  time  from  the  physical 
side  ;  but  no  conclusion  directly  affecting  the 
chemical  requirements  of  the  theory  has  as 
yet  been  arrived  at.  The  notion  of  an 
imperishable  atom  has  also  been  challenged, 
first  from  the  purely  theoretical  side  regarding 
the  atom  as  a  mechanism  capable  of  reaching 
a  point  of  internal  instability  when  a  certain 
degree  of  complexity  has  been  attained,  and 
then  from  the  observational  side  owing  to 
the  discovery  that  one  of  the  elements, 
radium,  is  capable  of  giving  rise  to  another 
element,  helium,  by  a  process  of  spontane- 
ous disintegration  (Rutherford;  Ramsay  and 
Soddy,  1903).  Here  we  are,  therefore, 
brought  again  directly  into  the  domain  of 


136  CHEMISTRY 

Chemistry ;  and  the  alchemical  dream  of 
"  transmutation  "  (p.  26)  has,  at  least  in  this 
case,  been  realized.  But  the  consideration  of 
this  subject  belongs  to  a  late  chapter  in  the 
history  of  Chemistry,  and  the  reader  must 
accept  this  brief  statement  by  way  of  pre- 
liminary introduction  to  the  newly  developed 
science  of  Radioactivity.  (See,  for  instance, 
the  volume  on  "  Matter  and  Energy,"  by 
Mr.  F.  Soddy,  in  this  Library.) 


CHAPTER   VI 

SYMBOLS    AND    NOTATION — ATOMS    AND    MOLE- 
CULES  ATOMIC  AND  MOLECULAR  WEIGHTS 

THE  DEFINITENESS  OF  CHEMICAL  COM- 
BINATION BY  VOLUME — THE  HYPOTHESIS 
OF  AVOGADRO 

Symbols  and  Notation. — The  transition  from 
the  crude  and  primitive  notions  concerning 
the  chemical  atom  to  the  current  conception 
and  use  of  the  atomic  theory  represents  a 
period  of  more  than  a  century.  It  was  a 
period  teeming  with  interest  from  the  histori- 
cal point  of  view ;  but  we  must  perforce  pass 
over  the  developmental  phases  and  deal 
with  the  science  of  Chemistry  as  it  stands 
to-day  in  the  light  of  the  theory.  And, 
before  any  substantial  progress  can  be  made, 
it  is  necessary  that  the  reader  should  master 
at  least  the  rudiments  of  chemical  language — 
they  are  simple,  easy  to  understand,  and 
replete  with  meaning.  Our  alphabet  is  no 
more  than  a  set  of  shorthand  symbols  for  the 
atoms  of  the  chemical  elements.  Symbols 
137 


138  CHEMISTRY 

in  the  form  of  letters  of  the  alphabet  have 
already  been  made  use  of  in  the  last  chapter 
in  order  to  give  general  expression  to  the 
conception  of  equivalence ;  all  that  is  now 
required  of  the  reader  is  to  attach  a  specific 
meaning  to  these  symbols.  The  chemical 
symbol  as  at  present  used  is  the  initial  lettei 
or  letters  in  the  name  of  the  element,  and  thia 
symbol  stands  for  one  atom  of  the  element. 
There  is  no  more  mystery  about  this  than  in 
writing  W.  for  Walter  and  Wm.  for  William ; 
the  only  difference  is  that  there  may  be 
multitudes  of  Williams  and  Walters,  and  that 
each  individual  is  distinguished  by  the  addi- 
tion of  one  or  more  names.  But  we  require 
no  binomial  system  in  Chemistry,  because 
each  symbol  stands  for  one  element  only, 
and  for  one  definite  weight  of  that  element 
— the  weight  of  the  atom  with  reference  to 
some  common  standard. 

The  chemical  elements  have  not  been 
named  on  any  coherent  system  ;  some  of  the 
names  are  survivals  from  a  remote  past,  while 
those  isolated  in  later  times  have  received 
names  indicating  either  some  characteristic 
property,  the  natural  source,  or  the  country 
in  which  the  isolation  was  effected.  All  that 
can  be  said  is  that  the  names  of  the  more 
recently  discovered  metals  have  been  given  the 


SYMBOLS    AND    NOTATION      139 

terminal  syllable  um.  Each  name  (Latinized 
in  a  majority  of  cases)  is  contracted  into 
an  initial  letter,  or,  if  several  elements 
have  the  same  initials,  into  two  letters.  All 
the  elements  named  in  the  previous  chapters 
could  have  been  made  to  tell  their  story  more 
concisely  if  symbols  had  been  used.  A  few 
examples  will  serve  to  illustrate  the  use  of 
the  chemical  alphabet.  Thus  H,  O,  N,  F, 
I  stand  respectively  for  one  atom  of  the 
non-metals  hydrogen,  oxygen,  nitrogen,  fluor- 
ine, and  iodine.  We  cannot  use  C  for  chlorine, 
because  there  are  several  other  elements 
beginning  with  C,  so  we  write  Cl  for  chlorine, 
C  for  carbon,  Cu  for  copper  (cuprum),  Ca 
for  calcium,  and  so  forth.  We  use  Br  for 
bromine  because  B  stands  for  boron ;  Bi  for 
bismuth,  and  Ba  for  barium.  S  stands  for 
sulphur,  Si  for  silicon,  and  Se  for  selenium. 
Iron  is  Fe  (ferrum),  mercury  or  quicksilver 
is  Hg  (hydrargyrum),  silver  is  Ag  (argentum), 
gold  is  Au  (aurum),  sodium  is  Na  (natrium), 
and  so  forth.  The  association  of  the  symbol 
with  the  name  of  the  element  is  only  a  matter 
of  practice,  and  no  great  strain  of  memory 
is  required.  A  complete  list  is  given  for 
reference  at  the  end  of  this  volume. 

Having  mastered  the   alphabet,   however, 
the  next  step  is  to  learn  how  to  use  it — ho\v 


140  CHEMISTRY 

to  read  a  chemical  formula.  Here,  again,  the 
principle  has  already  been  made  use  of  in 
order  to  illustrate  equivalence ;  the  only 
difficulty  that  the  uninitiated  reader  is  likely 
to  meet  is  the  confusion  arising  from  the  mix- 
ing up  of  chemical  symbols  with  algebraical 
expressions,  because,  in  chemical  notation, 
the  symbols  are  used  in  a  different  way.  Thus, 
in  algebra  AB  means  A  x  B,  but  the  chemical 
formula  AB  stands  for  a  chemical  compound 
in  which  one  atom  of  A  is  combined  with 
one  atom  of  B,  each  atom  having  its  specific 
weight.  So  ABC  in  Chemistry  stands  for  a 
compound  containing  one  atom  of  each  of  the 
elements  A,  B,  and  C,  and  not,  as  in  algebra, 
for  A  x  B  x  C.  In  algebra  the  numerical 
coefficient  multiplies  any  literal  symbols  that 
follow;  thus2A=2  x  A;  2AB=2  x  A  x  B. 
In  Chemistry  the  number  of  atoms  is 
multiplied  in  the  same  way,  so  that  2A 
means  two  atoms  of  A  ;  but  in  the  case  of 
compounds  the  whole  weight  of  the  compound 
is  multiplied  by  the  numerical  coefficient. 
Thus,  2AB  in  chemical  language  means  twice 
as  much  of  the  compound  as  is  represented 
by  AB,  SAB  three  times  as  much,  and  so 
forth.  It  is  as  though  we  wrote  the  equation 
2AB  =  2( A  -f-  B)  in  which  the  +  sign  is  made 
to  stand  for  chemical  combination.  The 


SYMBOLS    AND    NOTATION      141 

number  of  individual  atoms  in  compounds 
is  indicated  by  a  small  number  placed  below 
the  symbol,  to  avoid  confusion  with  the 
algebraical  system  of  indicating  the  "  power  " 
by  a  small  number  placed  above  the  symbol. 
Thus  A2B3  means  AxAxBxBxB; 
A2B3  means  a  compound  containing  two 
atoms  of  A  combined  with  three  atoms  of  B. 
A  general  chemical  formula  for  a  compound 
containing  three  elements  would  be  AxByCz ; 
and  the  multiplication  of  the  whole  weight 
represented  by  this  formula  would  be 
2AxByCz,  or  3AxByCz,  or  n  AxByCz. 

Atoms  and  Molecules. — The  congeries  of 
atoms  which  elements  form  when  they  com- 
bine with  themselves  or  with  the  atoms  of 
other  elements  are  termed  molecules.  This 
is  a  point  of  fundamental  importance  in 
modern  chemical  language,  because  it  is 
impossible  on  this  system  to  have  an  atom 
of  a  compound,  although  we  might  conceive 
the  existence  of  an  atom  of  an  element.  The 
justification  for  this  distinction  will  be  given 
immediately  ;  but  we  are  now  concerned  only 
with  notation,  and  it  will  be  sufficient  to  point 
out  to  the  student  of  the  history  of  Chemistry 
that  immense  confusion  was  caused  in  the 
early  period  of  the  atomic  theory  through  the 
failure  to  recognize  that  atoms  and  molecules 


142  CHEMISTRY 

were  different  things.  The  atom  is  the 
smallest  weight  of  an  element  that  enters 
into  the  composition  of  the  molecule  of  a 
compound,  so  that  while  A  -f  B  stands  for 
one  atom  of  A  mixed  (not  combined)  with  one 
atom  of  B,  AB  stands  for  one  molecule  of  the 
resulting  compound,  2  AB  for  two  molecules, 
and  so  forth.  And,  since  an  atom  can  com 
bine  with  one  or  more  similar  atoms  to  form 
a  molecule  of  the  element,  A3  stands  for 
one  diatomic  molecule  of  A,  A3  for  one 
triatomic  molecule ;  and  2A2,  3A2,  2A3,  etc., 
stand  in  these  cases  for  two,  three,  and 
two  of  the  molecules  represented  by  A2 
and  A3  respectively.  Moreover,  the  molecule 
of  a  compound  may  in  some  cases  combine 
with  one  or  more  similar  molecules  to  form 
compounds  of  double,  treble,  etc.,  molecular 
weight ;  and  this  state  of  affairs  is  conveni- 
ently represented  by  such  formulae  as  (AB)2, 
(AB)3,  etc.,  which  therefore  have  a  meaning 
different  from  that  expressed  by  2AB  and 
SAB,  which  indicate  two  and  three  distinct 
molecules  of  AB,  whereas  (AB)2,  (AB)3,  mean 
A2B2,  A3B3,  i.e.,  one  molecule  in  each  case. 

It  may  be  of  interest  to  point  out  that  the 
introduction  of  the  atomic  theory  and  the 
clear  recognition  of  the  distinction  between 
atoms  and  molecules  has  done  more  than  any 


ATOMS    AND    MOLECULES        143 

other  conception  to  bring  the  kindred  sciences 
of  Chemistry  and  Physics  into  intimate  associa- 
tion. In  the  direction  of  both  the  higher  and 
the  lower  limits  of  divisibility  of  matter,  the 
chemist  and  the  physicist  join  hands,  since 
physical  properties  are  associated  with  mole- 
cular constitution  and  chemical  properties 
with  the  chemical  atom ;  while  the  newer 
researches  into  the  constitution  of  the  atom 
are  laying  a  yet  deeper  foundation  for  both 
sciences. 

Atomic  and  Molecular  Weights. — Since  a 
molecule  is  composed  of  atoms,  its  weight 
as  a  concrete  particle  referred  to  the  same 
standard  as  that  to  which  are  referred  the 
weights  of  the  atoms  must  be  the  sum  of  the 
weights  of  the  atoms  of  which  it  is  composed. 
But,  in  view  of  the  principle  of  multiple 
equivalence,  it  is  evident  that  the  determina- 
tion, either  by  analysis  or  synthesis,  of  the 
relative  proportions  by  weight  of  the  elements 
contained  in  a  compound  does  not  decide  the 
question  of  the  actual  weight  either  of  the 
atoms  or  of  the  molecule.  Let  us  consider 
one  of  the  previous  cases.  One  part  of  hydro- 
gen combines  with  8  parts  of  oxygen  to  form 
9  parts  of  water.  The  simplest  representa- 
tion of  this  fact  is  the  symbol  HO,  which  now 
stands  for  a  molecule  of  water  with  a  molecular 


144  CHEMISTRY 

weight  of  9  referred  to  hydrogen.  Since  23 
parts  of  sodium  displace  and  are  therefore 
equivalent  to  1  part  of  hydrogen  (p.  124), 
the  corresponding  oxide  would  be  NaO  with 
the  molecular  weight  23  +  8  =  31.  In  sodi- 
um peroxide,  11 J  parts  of  sodium  are  com- 
bined with  8  parts  of  oxygen  (p.  125) — what 
then  is  the  atomic  weight  of  sodium,  11 J  or 
23  ?  If  11 J,  then  the  formula  for  the  first 
oxide  must  be  Na2O  =  (11 J  x  2)  +  8  =  31, 
and  of  the  peroxide  NaO  =  11 J  +  8  =  19J  to 
meet  the  arithmetical  requirements.  But 
11 J  cannot  be  taken  as  the  atomic  weight  of 
sodium  in  view  of  the  fact  that  twice  that 
weight  is  the  smallest  quantity  which  is 
equivalent  to  the  standard  unit  of  hydrogen. 
If  we  take  23  as  the  atomic  weight,  then  the 
formula  of  the  peroxide  becomes  NaO2  =  23 
+  (8x2),  and  the  molecular  weight  39.  And 
so,  on  the  23-sodium  scale,  the  lowest  admis- 
sible atomic  weight  for  sodium,  we  have  the 
two  oxides  NaO  and  NaO2,  with  the  atomic 
weight  of  oxygen  taken  as  8.  But  this  view 
of  the  composition  of  the  oxides  introduces 
another  equivalent  for  oxygen,  viz.,  1C,  since 
the  equivalents  are  respectively  23  :  8  and 
23  :  16  ;  and  the  question  now  assumes  the 
form — which,  if  either,  is  the  atomic  weight  of 
oxygen  ?  If  8,  then  the  formula  of  water  is 


ATOMS    AND    MOLECULES        145 

as  stated  above,  HO  =  9  ;  if  16,  the  formula 
must  be  H2O  =18.  On  this  same  scale,  the 
oxides  of  sodium  obviously  become  Na2O  and 
Na2O2  respectively. 

This  last  alternative  now  raises  another 
point.  In  the  peroxide,  the  ratio  Na :  O 
is  23  x  2  :  16  x  2,  i.e.,  46  :  32  ;  and  this  can, 
of  course,  be  halved  while  still  maintaining 
the  atomic  weight  of  16  for  oxygen.  The 
formula  then  becomes  NaO.  Moreover,  with 
the  same  atomic  weight  for  oxygen,  the 
formula  of  water  might  be  H2O,  H4O2,  or, 
generally  H2nOn  without  upsetting  the  ratio 
or  violating  the  fundamental  canon  of  an 
indivisible  atom.  It  is  clear,  therefore,  that 
the  equivalent  may  or  may  not  represent  the 
weight  of  the  atom ;  and  it  is  equally  plain 
that  the  answer  to  the  questions : — which  of 
the  equivalents  is  to  be  taken  as  the  weight  of 
the  atom  ? — how  many  atoms  are  there  in  the 
molecule?  cannot  possibly  be  given  without 
further  independent  evidence. 

It  will  be  seen  that  the  last  question  could 
be  answered  so  far  as  concerns  the  hydrogen  if 
it  were  possible  to  ascertain  the  relative  weight 
of  the  molecule  of  water,  because  we  should 
then  know  whether  it  was  9, 18,  or  9n  times  as 
heavy  as  an  atom  of  hydrogen.  We  might 
thus  find  out  how  many  atoms  of  hydrogen 


146  CHEMISTRY 

the  molecule  actually  contained,  because,  the 
ratio  being  always  1  :  8,  a  molecular  weight 
of  9  would  indicate  that  it  contained  one 
atom  of  hydrogen,  a  molecular  weight  of  18 
that  it  contained  two  atoms  of  hydrogen,  and 
so  forth.  But  the  question  of  the  number  of 
oxygen  atoms  in  the  molecule  is  still  left  open, 
even  when  the  number  of  hydrogen  atoms 
has  been  found,  because,  supposing  we  had 
found  the  molecular  weight  to  be  18  and  the 
number  of  hydrogen  atoms  2,  the  molecular 
formula  might  be  H2O  with  one  atom  of 
oxygen  of  atomic  weight  16,  or  H2O2  with 
two  atoms  of  oxygen  of  atomic  weight  8. 
Here,  again,  further  information  is  required 
before  it  can  be  definitely  decided  whether 
the  molecule  of  water  contains  3  or  4  atoms. 
In  the  early  days  of  the  atomic  theory,  no 
such  information  was  obtainable — or,  to  state 
the  case  accurately,  such  information  was 
available,  but  it  had  not  been  utilized  ;  the 
far-reaching  consequences  of  the  study  of 
chemical  combination  between  elements  and 
compounds  in  the  gaseous  state  had  not  been 
fully  realized.  As  a  result  of  this  lack  of 
evidence,  equivalent  weights  were  often  taken 
for  atomic  weights  ;  no  direct  method  for  the 
determination  of  the  value  of  n  in  the  equa- 
tion n  x  equivalent  =  atomic  weight  had  been 


ATOMS    AND    MOLECULES        147 

found.  Methods  of  determining  molecular 
weights  had  not  been  applied  in  Dalton's 
time — the  distinction  between  atoms  and 
molecules  had  not  been  clearly  appreciated. 

The  Definiteness  of  Chemical  Combination 
by  Volume. — With  the  exception  of  the  rela- 
tions between  the  weights  and  volumes  of 
hydrogen  and  oxygen  in  water  (p.  88), 
chemical  combination  by  weight  has  alone 
been  considered  up  to  this  stage.  Now,  the 
additional  evidence  required  in  order  to  decide 
definitely  the  relative  weight  of  the  atom  of  an 
element,  when  the  relative  weight  of  the  mole- 
cule of  a  compound  containing  that  element 
has  been  ascertained,  is  supplied,  as  hinted 
above,  by  the  study  of  chemical  combination 
by  volume.  This  presupposes  that  we  are 
dealing  with  elements  or  compounds  which 
are  or  can  be  made  gaseous — it  is  bulk  as 
distinguished  from  mass  that  has  now  to  be 
dealt  with.  But,  before  this  part  of  the 
subject  can  be  considered,  the  reader  must 
be  reminded  of  certain  elementary  physical 
principles.  All  gases  expand  when  heated, 
or  when  the  pressure  upon  them  is  reduced  ; 
they  contract  when  cooled,  or  when  the  pres 
sure  is  increased.  This  is  common  knowledge  ; 
but  it  can  be  stated  in  more  precise  terms.  The 
actual  change  in  volume  which  takes  place  on 


148  CHEMISTRY 

heating  or  cooling  is  2^3  per  degree  Centigrade 
— a  given  volume  of  gas  measured  at  0°  and  the 
pressure  remaining  constant,  increasing  by  this 
fraction  of  its  bulk  on  being  heated,  and 
diminishing  to  the  same  extent  on  being  cooled 
through  1°C.  So  that  at  -273°,  i.e.,  273°  below 
the  freezing-point  of  water,  the  gaseous  state  is 
presumably  non-existent.  That  point  is  known 
as  the  absolute  zero  ;  it  is  a  mathematical  abs- 
traction, and  has  never  yet  been  reached,  but 
recent  researches  on  the  liquefaction  of  the  gases 
by  Dewar  and  Onnes  have  brought  us  within 
three  or  four  degrees  of  it.  The  measurement 
of  the  coefficient  of  expansion  of  gases  is  due 
to  Charles  and  Gay-Lussac,  and  the  "  law  " 
thus  discovered  is  generally  associated  with 
the  name  of  Charles. 

The  effect  of  pressure  upon  gases  is  expressed 
quantitatively  by  the  statement  that  the 
volume  of  a  gas,  the  temperature  remaining 
constant,  varies  inversely  as  the  pressure. 
This  means  that,  if  the  pressure  is  doubled, 
the  volume  is  halved ;  if  the  pressure  is 
halved,  the  volume  is  doubled,  and  so  forth. 
In  mathematical  terms  : — Pressure  x  Volume 
=  Constant.  This  is  known  as  the  law  of 
Boyle  (1626-1691). 

Neither  the  law  of  Charles  nor  the  law  of 
Boyle  is  obeyed  absolutely  by  all  gases ; 


VOLUMETRIC    COMBINATION    149 

there  are  departures  depending  primarily  upon 
chemical  idiosyncrasies — upon  the  specific 
nature  of  the  element  or  compound.  There 
are  gases  which  approach  perfection,  and 
there  are  others  which  are  imperfect  gases. 
The  laws  in  their  most  generalized  form  are 
physical,  but  in  their  detailed  application 
they  are  equally  chemical.  For  the  present 
purpose,  however,  it  is  sufficient  to  regard 
them  as  purely  physical.  It  is  evident  that 
in  dealing  with  matter  so  sensitive  as  are 
gases  to  the  influences  of  temperature  and 
pressure,  the  conditions  must  be  specified  ; 
no  comparison  can  be  made  between  volumes 
of  gases  unless  the  conditions  are  comparable. 
Theoretically,  any  selected  temperature  and 
pressure  might  be  adopted  ;  but  practically 
it  is  convenient  to  suppose  that  the  volumes 
are  measured  at  0°C,  and  at  the  average 
atmospheric  pressure  of  760  mm.  of  mercury. 
Of  course,  the  measurements  need  not  be, 
and  in  fact  very  seldom  are,  made  at  this 
temperature  and  pressure  ;  in  practice  the 
volume  is  measured,  and  the  temperature 
and  the  pressure  (as  indicated  by  the  barom- 
eter column)  recorded.  Then,  by  the  laws 
of  Charles  and  Boyle,  the  volume  is  reduced 
by  calculation  to  the  volume  which  the  gas 
would  occupy  at  0°C.  and  760  mm. ;  the 


150  CHEMISTRY 

so-called  normal  temperature  and  pressure. 
Of  course,  the  method  is  imperfect ;  it  ignores 
the  idiosyncrasies  of  the  gases ;  but,  for 
practical  purposes  in  comparing  one  gas 
with  another,  the  errors  thus  included  in  the 
calculation  are  too  small  to  seriously  affect 
the  result.  The  reader  will  now  understand 
why  in  comparing  gases  it  is  a  sine  qua  non 
that  the  conditions  should  be  comparable. 

Now,  when  chemical  combination  takes 
place  between  gases,  it  is  found  that  there  is  as 
much  definiteness  about  the  process  as  there 
is  about  combination  by  weight.  The  ratios 
between  the  combining  volumes  are  as  fixed 
as  are  the  combining  weights ;  and  the 
numbers  expressing  these  ratios  are  always 
simple,  such  as  1:1,  1:2,  1:3,  etc.  And 
the  same  numerical  simplicity  is  observed 
when  the  volume  of  the  product  is  compared 
with  the  volumes  of  the  components.  Thus, 
there  may  be  combination  between  equal 
volumes  giving  two  volumes  of  the  product : 
1+1=2.  But  the  rules  of  arithmetic  are 
not  always  obeyed  ;  one  volume  may  com- 
bine with  two,  giving  rise  to  two  volumes  of 
product,  or  one  volume  may  combine  with 
three,  giving  rise  to  two  volumes  of  product. 
In  these  last  cases,  there  is,  of  course,  shrink- 
age or  contraction.  But  it  will  be  noticed 


VOLUMETRIC    COMBINATION    151 

that,  here  also,  the  numerical  relationships 
are  simple.  All  that  need  be  considered  is 
the  ratio  between  the  volume  of  the  mixed 
gases  before  and  that  of  the  product  after 
combination.  The  results  then  come  out  in 
this  way:  (1+1=)2:2;  (2-fl=)3:2; 
(3+  1  =  )4  :  2,  etc.,  so  that  in  the  first  case 
there  would  be  no  contraction  after  com- 
bination ;  in  the  second  case  there  would  be 
contraction  to  the  extent  of  |  of  the  volume 
of  the  mixed  gases,  and  in  the  third  case 
contraction  to  the  extent  of  J. 

These  are  observed  facts — they  were  worked 
out  with  the  greatest  skill  by  Gay-Lussac 
(1778-1850),  and  the  law  of  definite  volume 
combination  is  associated  with  his  name. 
A  few  examples  will  give  greater  precision 
to  the  conception  of  the  principle.  The  halo- 
gens combine  with  hydrogen  (p.  102).  If  the 
volumes  of  the  elements  are  measured  before, 
and  of  the  products  after  combination — all 
under  comparable  conditions — it  is  found  that 
equal  volumes,  say  of  hydrogen  and  chlorine, 
combine  to  give  two  volumes  of  hydrogen 
chloride,  1+1=2;  there  is  no  contrac- 
tion. Hydrogen  and  oxygen  are  contained 
in  water  in  the  proportion  of  two  volumes  of 
the  former  to  one  of  oxygen  (p.  88).  If  the 
water  resulting  from  the  combination  of 


152  CHEMISTRY 

measured  volumes  of  hydrogen  and  oxygen  is 
measured  in  the  form  of  steam  at  100° 
(because  water  is  solid  at  0°  and  760  mm.) 
and  compared  with  the  volume  of  its  com- 
ponents under  similar  conditions,  it  is  found 
that  it  occupies  the  same  space  as  the  two 
volumes  of  hydrogen.  So  that  in  this  case 
three  volumes  shrink  to  two — a  contraction 
of  one  third. 

In  many  cases,  although  compounds  con- 
taining certain  elements  are  well  known  and 
perfectly  definite,  yet  it  is  difficult  or  even 
impossible  to  form  such  compounds  by  direct 
combination.  For  example,  nitrogen  forms 
with  hydrogen  a  compound  which  is  the  familiar 
substance,  ammonia,  contained  in  "  smelling 
salts  " — a  pungent  smelling,  strongly  basic 
compound,  gaseous  at  ordinary  temperatures, 
liquid  below  —  34°  and  solid  at  -  -  77°.  It 
is  possible  but  not  easy  to  bring  about  direct 
combination  between  nitrogen  and  hydrogen 
in  such  a  way  as  to  measure  the  volume 
relationships.  In  this  case,  which  is  typical 
of  many  others,  it  is  much  easier  to  decompose 
a  measured  volume  of  ammonia  gas,  say  by 
a  stream  of  electric  sparks,  and  to  measure 
the  resulting  elements.  This  is  the  analytical 
as  distinguished  from  the  synthetical  method. 
It  is  thus  found  that  from  one  volume  of 


HYPOTHESIS    OF    AVOGADRO    153 

ammonia  there  are  produced  two  volumes  of 
a  mixture  of  nitrogen  and  hydrogen — the 
volume  is  doubled ;  and  it  is  therefore  con- 
cluded that  when  nitrogen  and  hydrogen  com- 
bine there  is  contraction  to  the  extent  of  one 
half.  And,  since  the  mixed  gases  resulting 
from  the  decomposition  contain  three  volumes 
of  hydrogen  to  one  of  nitrogen,  it  follows 
that  the  ratio  of  combining  volumes  is  3:1, 
and  the  ratio  of  the  volume  of  the  whole 
mixture  to  that  of  the  product  4:2.  Here 
again  the  validity  of  the  Gay-Lussac  law 
is  noticed.  Furthermore,  the  relative  weights 
of  hydrogen  and  nitrogen  in  ammonia  are 
3  : 14,  so  that  the  equivalent  of  nitrogen  in 
this  compound  is  4|,  that  being  the  weight 
which  combines  with  one  part  of  hydrogen. 

From  the  consideration  of  such  cases  there 
arise  almost  spontaneously  the  questions : 
— Is  there  any  relationship  between  definite- 
ness  of  combination  by  weight  and  by 
volume  ?  If  such  relationship  exists,  is  it 
explicable  in  terms  of  the  atomic  theory  ? 
Does  it  enable  us  to  state  definitely  how 
many  atoms  of  each  element  are  present  in 
the  molecule  of  a  compound  when  the  relative 
weight  of  the  latter  is  known  ? 

The  Hypothesis  of  Avogadro. — The  answer 
to  all  these  questions  can  now  be  given.  In 


154  CHEMISTRY 

the  first  place,  there  is  an  obvious  relationship 
between  combination  by  weight  and  by 
volume.  If,  as  stated  above,  one  volume  of 
chlorine  combines  with  an  equal  volume  of 
hydrogen,  and  if,  as  also  stated  (p.  119),  the 
ratio  of  combination  by  weight  is  1  :  35-2, 
it  follows  that  one  volume  of  chlorine  weighs 
35-2  times  as  much  as  an  equal  volume  of 
hydrogen.  Again,  in  the  case  of  water,  two 
volumes  of  hydrogen  combine  with  one 
volume  of  oxygen  and  the  weights  are  1 :  8, 
so  that  8  parts  by  weight  of  oxygen  occupy 
half  the  space  occupied  by  one  part  of  hydro- 
gen. For  equal  volumes,  therefore,  we  have 
the  relative  weights  J  :  8,  which  means  that 
oxygen,  referred  to  hydrogen  as  unity,  is  16 
times  heavier,  bulk  for  bulk,  under  comparable 
conditions.  And,  yet  again,  in  ammonia 
three  volumes  of  hydrogen  combine  with  one 
volume  of  nitrogen,  and  the  relative  weights 
are  1  :  4f ;  from  which  it  follows  (since  4f 
parts  of  nitrogen  occupy  J  the  space  occupied 
by  1  part  of  hydrogen)  that  the  relative  weights 
of  equal  volumes  are  J  :  4f ,  which  means, 
when  expressed  in  whole  numbers,  that  nitro- 
gen is,  bulk  for  bulk,  14  times  heavier  than 
hydrogen.  All  this  amounts,  therefore,  to  the 
statement  that  the  densities  of  the  elements 
chlorine,  oxygen  and  nitrogen  are,  on  the 


HYPOTHESIS    OF   AVOGADRO    155 

hydrogen  scale,  35-2,  16  and  14  respectively. 
The  relationship  between  weight  and  volume 
thus  conies  out  in  the  form  that  the  densities 
of  the  gaseous  elements  are  equal  to  or  are 
simple  multiples  of  the  equivalents  : — chlorine, 
35-2  x  1 ;  oxygen,  8x2;  nitrogen,  4|  x  3. 
These  facts  can,  of  course,  be  and  have  been 
ascertained  experimentally,  not  only  by  deter- 
mining the  relative  weights  of  the  elements 
present  in  the  respective  compounds  by 
analysis,  but  also  by  direct  weighing  of  the 
gases  under  comparable  conditions-^-i.e.,  by 
determining  their  vapour  densities. 

Turning,  in  the  next  place,  from  the  ele- 
ments to  the  compounds  resulting  from  their 
union,  the  densities  can  also  be  found  from 
the  quantitative  composition  and  the  volume 
occupied  by  the  gas,  or  by  direct  weighing. 
Thus,  36-2  parts  of  hydrogen  chloride  resulting 
from  the  combination  of  1  part  of  hydrogen 
with  35-2  parts  of  chlorine,  occupy  twice  the 
volume  of  the  hydrogen,  as  just  explained. 
The  density  of  hydrogen  chloride  is,  therefore, 
I  of  36-2  =  18-1.  The  9  parts  of  water 
resulting  from  the  combination  of  1  part  of 
hydrogen  with  8  parts  of  oxygen  occupy  the 
same  volume  as  the  hydrogen,  so  that  the 
density  of  water  vapour  is  9.  The  5|  parts 
of  ammonia  resulting  from  the  combination  of 


156  CHEMISTRY 

1  part  of  hydrogen  with  4|  parts  of  nitrogen 
occupy  f  of  the  volume  of  the  hydrogen  (be- 
cause three  volumes  of  hydrogen  and  one 
volume  of  nitrogen  contract  to  two  volumes 
of  ammonia  on  combination),  so  that  the  ratio 
of  the  weights  of  equal  volumes  of  hydrogen 
and  ammonia  is  f :  5  j  =  1 :  8} ;  in  other 
words,  the  density  of  ammonia  on  the  hydro- 
gen scale  is  8J. 

Looking  at  these  results  in  a  broad  way,  it 
will  be  seen,  in  the  first  place,  that  we  have 
really  under  consideration  two  distinct  classes 
of  particles  or  aggregates — elementary  and 
compound.  The  former  might  be  atoms  or 
congeries  of  similar  atoms  ;  the  latter  must 
of  necessity  be  molecules.  It  will  be  seen, 
also,  that  the  densities  of  the  elements  bear 
some  definite  relationship  to  the  weights  of 
the  atoms  ;  but  what  that  relationship  really 
is  could  not  be  stated  in  numerical  terms 
unless  we  knew  the  relative  numbers  of  atoms 
in  equal  volumes.  If  it  were  assumed,  for 
example,  that  these  densities  did  actually 
represent  the  respective  weights  of  the  atoms, 
then  we  are  confronted  with  the  apparent 
paradox  that  the  weight  of  the  molecule  of  a 
compound  is  less  than  the  sum  of  the  atomic 
weights  of  its  components.  On  the  assump- 
tion that  the  atoms  of  chlorine,  oxygen,  and 


HYPOTHESIS    OF    AVOGADRO    157 

nitrogen  weigh  35-2,  16  and  14,  the  minimum 
weight  of  the  molecules  of  hydrogen  chloride, 
water  vapour,  and  ammonia  must  be  36-2, 
18  and  17  respectively.  But  the  densities 
are  18-1,  9  and  8J.  The  weight  of  water 
vapour  contained  in  a  volume  which  contains 
1  part  of  hydrogen  or  16  parts  of  oxygen  is  9  ! 
Clearly  9  cannot  be  the  weight  of  the  molecule 
of  water,  or  16  cannot  be  the  atomic  weight  of 
oxygen.  The  weight  of  ammonia  which  goes 
into  the  same  space  as  1  part  of  hydrogen  or 
14  parts  of  nitrogen  is  8| !  The  molecular 
weight  of  ammonia  must  be  greater  than  8J, 
or  the  atomic  weight  of  nitrogen  must  be  less 
than  14.  The  weight  of  the  whole  cannot  be 
less  than  the  weight  of  its  parts,  unless  matter 
is  annihilated  during  chemical  combination. 
The  vapour  densities  of  elements  and  com- 
pounds when  compared  do  not  decide  the 
molecular  weights  of  the  latter  any  more 
than  do  the  combining  weights  or  equivalents 
of  the  elements  contained  in  the  compound 
unless  there  is  made  a  certain  assumption  or 
hypothesis. 

Such  an  assumption  was  made  and  enun- 
ciated as  a  hypothesis  in  1811  by  the  Italian 
physicist  Amadeo  Avogadro.  In  the  light  of 
this  hypothesis,  all  the  above  discrepancies 
disappear,  and  the  vapour  density  of  a  com- 


158  CHEMISTRY 

pound  can  be  made  to  decide  the  weight  of 
the  molecule.  According  to  Avogadro,  equal 
volumes  of  all  gases,  elementary  and  com- 
pound, contain,  under  comparable  conditions, 
the  same  number  of  molecules.  It  will  be 
noted  that  the  particles  recognized  by  the 
hypothesis  are  molecules — not  atoms  ;  there 
is  thus  introduced  the  conception  of  the 
molecules  of  elements,  as  well  as  of  com- 
pounds. This  view  of  the  constitution  of 
gases  harmonizes  completely  with  those  physi- 
cal properties  which  find  quantitative  expres- 
sion in  the  laws  of  Charles  and  of  Boyle.  But 
this  side  of  the  subject  is  dealt  with  in  works 
on  Physics  under  the  kinetic  theory  of  gases, 
and  need  not  be  enlarged  upon  here.  Nearly 
half  a  century  elapsed  before  the  significance 
of  the  hypothesis  in  Chemistry  was  realized ; 
it  is  mainly  due  to  the  advocacy  of  the  late 
Prof.  Cannizzaro  in  1858  that  it  has  become 
the  foundation  of  modern  chemical  theory. 
Let  us  consider  some  of  the  previous  examples 
in  the  light  of  Avogadro's  conception. 

One  volume  of  hydrogen,  which  may  be 
taken  as  the  unit  of  comparison  and  assigned 
unit  weight,  combines  with  one  volume  of 
chlorine  of  weight  35-2  to  form  two  volumes 
of  hydrogen  chloride  of  weight  36-2.  The  35-2 
parts  of  chlorine  or  the  1  part  of  hydrogen 


HYPOTHESIS    OF   AVOGADRO    159 

occupy  half  the  volume  occupied  by  the 
product.  Therefore  the  latter — the  product 
volume — must,  by  hypothesis,  contain  twice 
as  many  molecules  as  there  were  of  chlorine 
or  of  hydrogen  in  the  original  mixture.  But 
the  molecules  of  chlorine,  although  only  half 
as  numerous  as  the  molecules  of  hydrogen 
chloride,  have  become  equally  distributed 
among  the  molecules  of  the  compound,  every 
one  of  which  contains  its  full  share  of  chlorine, 
i.e.,  35-2  parts  by  weight.  Each  molecule  of 
chlorine  has  accordingly  split  into  two  equal 
parts — it  is  the  molecule  which  has  divided 
and  not  the  atom,  and  the  atoms  are  the 
components  of  the  diatomic  molecule,  C12, 
which  is  composed  of  two  atoms  of  a  weight 
35-2  each.  By  precisely  similar  reasoning 
applied  to  the  hydrogen  molecule,  we  arrive 
at  the  conclusion  that  this  also  is  a  diatomic 
molecule,  H2,  composed  of  two  atoms  of  unit 
weight.  Therefore,  there  is  no  violation  of  the 
atomic  theory — the  atom  is  still  an  indivisible 
particle ;  the  combination  between  the 
elements  is  an  interchange  of  partners,  and 
not  a  simple  juxtaposition  of  atoms. 

From  the  same  point  of  view,  consider  the 
case  of  water.  The  oxygen  molecules  must 
distribute  themselves  uniformly  among  double 
the  number  of  water  molecules,  because  the 


160  CHEMISTRY 

water  vapour  occupies  double  the  space 
occupied  by  the  oxygen  which  enters  into  its 
composition.  Here  again,  it  is  the  molecule 
which  divides,  and  the  formula  of  the  molecule 
must  be  O2,  irrespective  of  the  question 
whether  the  atom  weighs  8  or  16.  So  also 
with  respect  to  ammonia,  a  given  volume  of 
nitrogen  distributes  itself  uniformly  through- 
out double  the  volume  of  ammonia,  because 
one  volume  of  nitrogen  combines  with  three 
volumes  of  hydrogen  to  form  two  volumes 
of  ammonia.  This,  in  terms  of  the  hypothesis, 
means  that  the  nitrogen  molecule  is  N2,  and 
that  it  divides  into  two  equal  parts  when  it 
combines  with  hydrogen. 

Now  sum  up  the  facts  and  arguments  set- 
ting out  from  the  observation  that  the 
densities  of  the  elements  under  consideration 
are  H  =  1,  Cl  =  35-2,  O  =  16,  N  =  14  :— 

(a)  1  part  by  weight  of  hydrogen  occupies 
half  the  volume  of  36-2  parts  of  hydrogen 
chloride  ;  therefore  2  parts  of  hydrogen  occupy 
the  same  volume  as  36-2  parts  of  hydrogen 
chloride. 

(b)  35-2  parts  of  chlorine  occupy  half  the 
volume  occupied  by  36-2  parts  of  hydrogen 
chloride ;     therefore   70-4    parts    of   chlorine 
occupy  the  same  volume  as   36-2   parts  of 
hydrogen  chloride. 


HYPOTHESIS    OF   AVOGADRO   161 

(c)  16   parts   of   oxygen   occupy   half   the 
volume  occupied  by  18  parts  of  water  vapour  ; 
therefore  32  parts  of  oxygen  occupy  the  same 
volume  as  18  parts  of  water  vapour. 

(d)  14  parts  of  nitrogen  occupy  half  the 
volume  occupied  by  17  parts  of  ammonia ; 
therefore   28   parts   of   nitrogen   occupy   the 
same  volume  as  17  parts  of  ammonia. 

To  compare  equal  volumes  of  elements  and 
compounds  we  have,  therefore,  to  double  the 
volumes  and,  by  implication,  the  weights  of 
the  elements.  Then  we  have  a  series  of 
weights  and  volumes  which  are  strictly 
comparable,  the  volumes  all  containing  the 
same  number  of  molecules.  On  this  scale  the 
hydrogen  standard  becomes  2,  and  the  vapour 
densities  of  the  other  elements  and  of  their 
compounds  do  represent  the  relative  molecular 
weights.  In  other  words,  the  volumes  occu- 
pied by  the  molecular  weights  of  elements 
and  compounds  are  equal,  and  as  this  holds 
good  for  all  elements  and  compounds  (with 
certain  exceptions  which  must  be  considered 
later)  and  irrespective  of  the  number  of  atoms 
which  form  the  compound  molecule,  we  arrive 
at  the  generalized  statement : — Vapour  density 
(referred  to  hydrogen  as  1)  x  2  =  molecular 
weight,  or  molecular  weight  -j-  2  =  vapour 
density  (rej erred  to  hydrogen  as  1).  And  so  the 


162  CHEMISTRY 

question  whether  the  relative  weight  of  the 
molecule  can  be  determined  by  ascertaining 
the  vapour  density  is  answered  definitely  in  the 
affirmative.  The  apparent  paradox  previously 
raised — that  the  molecule  of  a  compound  can 
weigh  less  than  the  sum  total  of  the  atoms 
of  its  components — disappears  when,  as  we 
now  find,  the  molecular  weight  of  hydrogen 
chloride  becomes  36-2,  of  water  18,  and  of 
ammonia  17. 


CHAPTER   VII 

THE  NUMBER  OF  ATOMS  CONTAINED  IN  A  MOLE- 
CULE  DISSOCIATION    AND    ASSOCIATION — 

AUXILIARY    METHODS    FOR    DETERMINING 

ATOMIC    AND    MOLECULAR    WEIGHTS THE 

LAW      OF     DULONG      AND      PETIT ATOMIC 

AGGREGATES    IN    SOLUTION 

The  Number  of  Atoms  Contained  in  a  Molecule. 
— The  chemical  molecule  may  be  regarded  in 
the  light  of  these  conclusions  as  a  weight  of 
matter  representing  in  the  case  of  compounds 
an  irreducible  minimum,  because  any  further 
division  must  obviously  lead  to  the  removal 
of  atoms,  i.e.,  to  decomposition,  and  the  com- 
pound, as  such,  ceases  to  exist.  And,  since 
the  weight  of  the  molecule  is  the  sum  total  of 
the  weights  of  its  component  atoms,  we  have 
now  to  face  the  next  question — whether,  the 
relative  weight  of  the  molecule  being  known, 
it  is  possible  to  decide  therefrom  the  numbers 
of  the  atoms  of  the  elements  of  which  it  is 
composed.  We  know,  for  instance,  that  a 
molecule  of  water  has  the  relative  weight  18, 
163 


164  CHEMISTRY 

and  that  it  contains  two  atoms  of  hydrogen 
because  it  contains  the  divisible  quantity  of 
that  element  represented  by  H2.  The  water 
molecule,  therefore,  contains  18—2=16  parts 
of  oxygen  ;  and  the  question  is — how  many 
atoms  of  oxygen  does  this  represent  ?  Similar 
questions  arise  concerning  all  the  compounds 
with  which  we  have  dealt. 

It  has  already  been  pointed  out  that  the 
formula  H2O2,  with  an  atomic  weight  of  8  for 
oxygen,  satisfies  the  arithmetical  require- 
ments ;  and  it  will  now  be  seen  that  it  also 
satisfies  the  molecular  requirements,  because 
the  molecule  of  oxygen  may  be  supposed  to 
consist  of  4  atoms,  in  which  case  its  formula 
would  be  O4,  and  the  splitting  of  the  oxygen 
molecule  would  be  represented  by  the  equa- 
tion O4  =  2  O2  The  answer  to  the  question 
now  raised  is  really  in  principle  a  very  simple 
one.  It  is  given  by  taking  a  consensus  of 
evidence — by  a  kind  of  Referendum  to  all 
the  known  gasifiable  compounds  of  the  ele- 
ment in  order  to  find  out,  by  comparing  the 
weights  of  the  element  contained  in  the  mole- 
cules of  the  various  compounds,  the  smallest 
weight  contained  therein.  That  weight  is 
reasonably  taken  to  be  the  irreducible  mini- 
mum of  the  element,  i.e.,  its  atomic  weight. 
It  corresponds  with  the  definition  of  the  atom 


ATOMIC   AGGREGATES  165 

previously  given  (p.  142)  —  the  smallest  weight 
of  the  element  which  enters  into  the  composi- 
tion of  the  molecule,  or  which  can  be  separated 
from  the  molecule  by  decomposition.  In  this 
way  we  find  that  the  smallest  weights  of 
chlorine,  oxygen,  nitrogen,  etc.,  contained 
in  the  molecules  of  any  of  their  compounds 
are,  respectively,  35-2,  16  and  14  ;  and  we  take 
these  as  the  weights  of  their  atoms  —  we  find 
the  value  of  n  in  the  equation  n  x  equivalent 
=  atomic  weight  (p.  146).  In  this  case  for 
Cl,  n  =  1  ;  for  O,  n  =  2  ;  and  for  N,  n  =  3. 
And  thus  we  are  enabled  to  write  the  formulae 
of  the  compounds  HC1,  H2O  and  NHa,  and 
to  indicate  the  actual  numbers  of  atoms 
composing  their  molecules.  Furthermore,  we 
can  represent  the  formation  of  these  com- 
pounds from  their  elements  by  equations 
which  are  both  chemically  and  arithmetically 
true  :  — 

H2  +  Cla  =  2  HC1  ;  2H2  +  Oa  =  2  HaO  ; 
N2+3H2  = 


It  has  already  been  claimed  that  the  sym- 
bolical language  of  Chemistry  is  full  of 
meaning.  The  reader  will  now  perceive  the 
wealth  of  truth  embodied  in  these  symbols. 
The  formula  for  a  chemical  compound, 
AxByCz,  stands  for  one  molecule  —  for  a  weight 


166  CHEMISTRY 

of  the  compound  made  up  of  the  sum  of  the 
weights  of  x  atoms  of  A,  y  atoms  of  B,  and  z 
atoms  of  C.  It  represents  also  a  weight  of  the 
compound  which  in  the  gaseous  state — if  it  is 
gasifiable — occupies  the  same  volume  as  the 
molecule  of  hydrogen,  i.e.,  H2.  It  will  be  seen, 
also,  that  the  symbols  for  the  atoms  of  those 
elements  which  possess  diatomic  molecules 
represent  half  volumes  ;  the  atomic  weights  of 
H,  Cl,  O,  N,  etc.,  all  occupy  the  same  volume 
in  the  gaseous  state.  The  law  of  definiteness 
of  combination  by  volume  (p.  147)  is  thus 
explained — the  atomic  weights  must  combine 
in  equal  volumes,  or  in  some  simple  multiple 
of  these  volumes. 

Dissociation  and  Association. — There  are 
certain  elements  and  compounds  whose  vapour 
densities  are  apparently  unconformable ; 
there  are  discrepancies  between  the  numbers 
obtained  for  their  atomic  or  molecular  weights 
and  the  numbers  deduced  from  the  law  of 
uniformity  of  atomic  and  molecular  volumes. 
But  this  nonconformity  is  not  paradoxical 
— on  the  contrary,  it  is  instructive  in  the 
highest  sense.  From  the  story  of  such  cases 
we  get  a  glimpse  into  new  principles.  Let 
us  consider  some  of  the  facts. 

The  atomic  weight  of  sulphur  as  deduced 
from  the  analysis  and  vapour  densities  of  its 


DISSOCIATION  AND  ASSOCIATION    167 

gaseous  compounds  is  32.  If  it  were  conform- 
able, its  molecular  weight  should,  therefore, 
be  64,  and  its  vapour  density  32.  In  fact, 
this  element  has  two  vapour  densities,  accord- 
ing to  the  temperature  at  which  the  vapour 
is  weighed.  At  its  boiling-point  under 
atmospheric  pressure  (448°),  its  vapour  density 
is  128  ;  and  at  1700°  its  vapour  density  is 
normal,  i.e.,  32.  Translate  these  facts  into 
terms  of  the  atomic-molecular  theory,  and 
consider  their  meaning.  At  448°  the  volume 
of  sulphur  vapour  which  occupies  the  space 
occupied  by  1  part  by  weight  of  hydrogen 
weighs  128,  and  its  molecular  weight  is  accord- 
ingly 256.  Similarly,  its  molecular  weight 
at  1700°  is  64.  There  is  really  no  mystery 
about  this.  If,  as  the  evidence  shows,  the 
atom  weighs  32,  then  at  448°  the  molecule 
must  contain  8  atoms,  i.e.,  32  x  8  =  256  ; 
the  formula  is  S8.  At  1700°  the  molecule 
must  for  the  same  reason  be  S2 ;  and,  as  the 
vapour  cools  down  from  1700°  to  448°,  the 
simpler  aggregate  of  2  atoms  condenses  to 
the  more  complex  aggregate  of  8  atoms.  This 
brings  out  the  principle  of  Dissociation  and 
Association — terms  which,  in  the  light  of  the 
foregoing  example,  should  be  self-explanatory. 
Again,  phosphorus — the  highly  combustible 
non-metallic  element  used  in  the  manufacture 


168  CHEMISTRY 

of  luteifer  matches — has  an  atomic  weight  of 
about  31,  as  determined  by  the  usual  methods. 
At  313°  the  vapour  density  gives  a  molecular 
weight  of  31  x  4  =  124,  and  the  molecule  is 
P4.  As  the  temperature  of  the  vapour  is 
raised,  the  P4  dissociates,  and  at  1700°  it  is 
largely  resolved  into  P2. 

In  a  similar  way,  and  by  means  of  modern 
appliances  which  are  described  in  works 
dealing  with  practical  Chemistry,  the  molecu- 
lar weights  of  many  other  elements  reveal 
this  principle  of  dissociation  and  association. 
Thus,  iodine  of  atomic  weight  about  127  at 
448°  is  normally  I2  ;  at  1700°  it  completely 
dissociates  into  I — the  molecule  and  the  atom 
at  this  temperature  are  the  same  ;  the  molecule 
is  monatomic.  Sodium,  zinc,  mercury,  etc., 
have  by  a  similar  method  been  shown  to  exist 
at  high  temperatures  as  Na,  Zn,  Hg,  i.e.,  as 
monatomic  molecules. 

So  also  with  regard  to  compounds.  Hydro- 
gen fluoride  (p.  102)  is  chemically  analogous 
to  hydrogen  chloride,  the  vapour  density  of 
which  accords  with  the  formula  HC1.  But  the 
vapour  density  of  the  fluoride  indicates  that 
below  30°  its  molecule  is  (HF)2,  i.e.,  H2F2, 
and  that  at  88°  dissociation  has  taken  place ; 
as  the  temperature  falls  association  takes 
place:— 2HF^H2F2.  The  reader  will  inci- 


DISSOCIATION  AND  ASSOCIATION    169 

dentally  notice  the  potency  of  our  symbolical 
language.  It  is  only  necessary  to  call  atten- 
tion to  a  certain  oxide  of  nitrogen,  to  the 
compound  which  phosphorus  forms  with 
chlorine  (phosphorus  pentachloride),  or  to  the 
compound,  ammonium  chloride,  formed  by 
the  direct  union  of  one  molecule  of  ammonia 
with  one  molecule  of  hydrogen  chloride,  as 
further  illustrations  of  the  principle  and  to 
write  the  story  of  their  dissociation  and 
association  by  means  of  such  schemes  as 
these:  N2O4,  ^±  2NO2 ;  PC16  ;±  PC13  +  C12 ; 
NH4C1  ^  NH3  +  HC1. 

The  only  information  not  included  in  the 
symbolical  representation  of  the  process  is 
the  temperature  at  which  dissociation  takes 
place  in  each  case,  and  the  influence  of  pres- 
sure upon  the  amount  of  dissociation.  The 
combination  between  compound  molecules, 
such  as  ammonia  and  hydrogen  chloride, 
furnishes  an  illustration  of  the  extended 
principle  of  equivalence  (p.  121),  since  17  parts 
of  ammonia  and  86-2  parts  of  hydrogen 
chloride  represent  equivalent  weights  of  these 
two  compounds. 

Auxiliary  Methods  for  determining  Atomic 
and  Molecular  Weights. — The  reader  who  has 
followed  the  development  of  the  principles 
of  chemical  science  up  to  this  point  will  now 


170  CHEMISTRY 

realize  that  the  methods  employed  for  the 
determination  of  the  relative  weights  of 
atoms  and  molecules  enable  us  to  state  defin- 
itely that,  under  such  or  such  conditions, 
such  or  such  atomic  aggregates  exist.  All 
the  gaseous  elements  referred  to  in  former 
chapters  form  diatomic  molecules.  Our  imag- 
inary being  of  the  second  chapter  who  was 
capable  of  following  the  gyrations  of  the 
"  particles  "  of  air  would,  in  fact,  see  vast 
crowds  of  twin  pairs  of  atoms  of  nitrogen, 
smaller  numbers  of  twin  pairs  of  atoms  of 
oxygen,  and  still  smaller  numbers  of  triatomic 
groups  constituting  the  molecules  of  water 
(H2O)  and  carbon  dioxide  (CO2).  And  he 
would  also  see  now  and  again  solitary  mona- 
tomic  nomads  of  certain  very  rare  gases 
(argon,  neon,  krypton,  xenon),  which  have 
already  been  stated  to  have  been  detected 
in  late  years  as  constituents  of  the  atmos- 
phere. But,  great  as  is  the  insight  into  the 
constitution  of  matter  gained  by  this  com- 
bination of  chemical  and  physical  methods  of 
attacking  the  problem,  it  must  be  realized 
that  the  results  are  true  only  within  the 
limiting  conditions  of  the  observations.  The 
state  of  aggregation  revealed  is  true  within 
the  ranges  of  temperature  and  pressure  avail- 
able for  the  determination  of  atomic  and 


AUXILIARY    METHODS          171 

molecular  weights ;  but  it  must  not  be  inferred 
that  no  further  disaggregation  is  possible  at 
extreme  temperatures  (say  celestial),  or  under 
the  influence  of  other  disintegrating  forces. 
Neither  can  it  be  asserted  that  in  the  other 
direction — in  the  passage  from  the  gaseous 
to  the  liquid  or  solid  state — there  is  no 
greater  complexity  than  that  indicated  by  the 
molecular  formula  as  ascertained  by  the 
above  methods. 

Take  the  case  of  the  elements.  Under 
infinitesimal  pressure,  i.e.,  in  a  vacuous  space 
containing  comparatively  few  molecules,  there 
is  physical  evidence  that  the  atoms  are  cap- 
able of  being  broken  down  by  an  electric 
discharge  into  smaller  particles — "  corpus- 
cles," or  "  electrons,"  or  whatever  they  may 
be  (p.  134).  But  this  ultra-dissociation  by 
ultra-chemical  means  in  no  way  conflicts 
with  the  views  concerning  the  atom  derived 
from  the  study  of  its  chemical  and  physical 
attributes  under  ordinary  conditions.  It  is 
supplementary  information  concerning  the 
inner  mechanism  of  the  atom  as  a  discrete 
particle  of  matter,  of  which  the  direct  bearing 
upon  the  nature  of  chemical  change  has  yet 
to  be  deciphered.  The  atom  may  be  knocked 
to  pieces  by  an  electric  discharge  in  a  high 
vacuum,  or  may  be  disintegrated  in  the 


172  CHEMISTRY 

atmosphere  of  the  hottest  stars  (Lockyer) ; 
but  in  the  course  of  all  the  ordinary  chemical 
transformations  which  matter  undergoes  on 
this  earth  it  may  still  be  regarded  as  the 
indivisible  particle. 

So  also  in  the  other  direction — in  the  way 
of  increased  complexity — the  molecule,  let 
us  say,  of  vaporous  water,  H2O,  may  not  be 
the  molecule  of  liquid  water,  which  is  cer- 
tainly (H2O)n.  The  value  of  n  cannot  be  said 
to  have  been  definitely  established  yet.  So 
generally  in  the  case  of  solids,  we  have  no 
criterion  of  the  state  of  molecular  aggregation 
based  on  vapour  density  determinations  only. 
The  three  oxides  of  iron,  for  example  (p.  124), 
have  the  formulae  Fe2O3  (rust),  Fe3O4  (scale), 
and  FeO  respectively ;  the  sulphide  (p.  128)  has 
the  formula  FeS,  and  the  iodide  (p.  73)  Fela. 
But  these  are  minimum  formulae — we  cannot 
convert  any  one  of  these  compounds  into 
vapour ;  and  the  atomic  weights  of  the 
elements  composing  these  molecules  have 
been  determined  independently  of  these 
particular  compounds.  The  molecules  of  the 
solid  compounds  may  be,  and  probably  are, 
multiples  of  these  formulae.  Then,  again,  in 
the  case  of  the  elements,  the  molecule  of 
the  liquid  or  solid  might  be  a  multiplex  aggre- 
gate of  gaseous  molecules.  Moreover,  there 


AUXILIARY   METHODS          173 

are  many  elements  which,  like  carbon,  for 
example,  cannot  be  gasified  at  any  manage- 
able temperature ;  and  there  are  elements 
which  are  not  only  non-volatile,  but  which 
form  no  gasifiable  compounds.  It  is  evident, 
therefore,  that  other  evidence  is  wanted ; 
the  method  based  upon  the  hypothesis  of 
Avogadro  is  inapplicable  in  such  cases  as 
those  referred  to  above — auxiliary  methods 
are  needed. 

The  Law  of  Dulong  and  Petit. — The  reader 
will  learn  from  Physics  that  different  kinds 
of  matter  require  different  quantities  of  heat 
to  raise  equal  weights  through  the  same  range 
of  temperature.  This  is,  therefore,  a  specific 
property  of  matter ;  and  the  quantity 
of  heat  required  to  raise  some  particular 
substance  through  some  definite  range  of 
temperature  is  said  to  be  the  specific  heat  of 
that  substance.  The  standards  in  use  are 
generally  1°C.  and  the  specific  heat  of  water 
as  unity ;  the  quantity  of  heat  required  to 
raise  one  gram  of  water  through  1°C.  is  known 
as  the  calorie.  We  cannot  deal  here  with 
the  physics  of  this  property  of  matter — why 
different  substances  should  require  different 
quantities  of  heat  to  produce  the  same  visible 
effect  upon  a  thermometer ;  neither  is  it 
necessary  to  describe  the  methods  of  deter- 


174  CHEMISTRY 

mining  specific  heat,  as  these  are  dealt  with  in 
works  on  Physics.  From  the  chemical  point 
of  view,  this  property  of  matter  can  be 
utilized  as  an  auxiliary  method  for  checking 
atomic  weights.  It  was  discovered  by  Dulong 
and  Petit  (1818)  that  if,  instead  of  comparing* 
the  specific  heats  of  equal  weights  of  the 
chemical  elements  in  the  solid  condition,  the 
specific  heats  of  weights  of  the  elements 
proportional  to  their  atomic  weights  are 
compared,  then  the  fact  is  revealed  that  all 
the  elements  have  the  same  atomic  heat.  In 
other  words,  the  property  in  question  is  a 
property  of  the  atom ;  the  atoms  all  require 
the  same  quantity  of  heat  to  raise  them 
through  the  same  range  of  temperature.  The 
specific  heats  of  the  elements  are  thus  inversely 
as  the  atomic  weights,  so  that  Atomic  Weight 
x  Specific  Heat  =  a  Constant.  The  value  of  that 
constant  is  between  6  and  7,  and  averages  6-4, 
so  that  6-4  -r  Specific  Heat  =  Atomic  Weight. 

This  law  is  only  approximative,  and  there 
are  deviations  from  it — some  to  a  considerable 
extent.  But  these  abnormalities  arise  from 
the  circumstance  that  in  the  case  of  certain 
elements  the  specific  heat  only  approaches  the 
normal  value  at  high  temperatures.  Passing 
over  these  exceptional  cases,  the  remainder 
of  the  elements  conform  closely  with  the  law* 


AUXILIARY    METHODS          175 

But,  although  the  latter  is  only  approximative, 
it  follows  from  the  relationship  between 
equivalents  and  atomic  weights  that  there 
is  a  wide  margin  within  which  the  numerical 
results  may  be  allowed  to  fluctuate.  The 
equivalents  are  exact — as  accurate  as  can  be 
determined  by  the  most  refined  and  delicate 
methods  of  analysis.  But,  after  we  have  ascer- 
tained the  equivalent,  its  relationship  to  the 
weight  of  the  atom  has,  as  already  pointed 
out,  to  be  settled  by  independent  evidence 
In  cases  where  the  molecular  weight  cannot 
be  determined  directly,  or  where  the  element 
forms  no  gasifiable  compound,  the  specific 
heat  gives  valuable  information.  The  whole 
point  to  be  decided  is  the  same  as  that  which 
arises  in  the  case  of  the  gaseous  elements,  viz., 
whether  the  equivalent  A  represents  the 
weight  of  the  atom,  or  whether  the  atomic 
weight  is  2A,  3A,  etc.  That  is  why  this 
auxiliary  method  based  on  the  law  of  Dulong 
and  Petit  admits  of  considerable  numerical 
latitude.  A  few  illustrations  will  enable  the 
reader  to  appreciate  its  value  :— 

The  vapour  density  of  mercury  on  the 
hydrogen  scale  is  100,  so  that  its  molecular 
weight  is  200.  If  the  molecule  consisted  of 
two  atoms,  the  atomic  weight  would  be  100. 
But  the  specific  heat  of  solid  mercury  is  -032 


176  CHEMISTRY 

(water  =  1)  and  6-4  -f-  -032=  200,  and  so  the 
atomic  weight  of  mercury  is  taken  as  200,  and 
that  is  one  of  the  reasons  why  the  molecule 
is  considered  to  be^monatomic  (p.  168).  Again, 
which  of  the  equivalents  of  iron  (p.  124) 
represents  the  weight  of  the  atom  ?  The 
specific  heat  of  iron  is  -112,  and  6-4  -f-  '112  = 
57-1.  None  of  the  three  equivalents,  21, 
18 1,  or  28,  represents  the  weight  of  the  atom, 
but  the  nearest  is  evidently  28  x  2  ;  and  it 
is  for  this,  among  other  reasons,  that  the 
atomic  weight  of  iron  is,  in  round  numbers, 
accepted  as  56.  Copper  forms  two  oxides, 
and  has,  therefore,  the  two  equivalents  31-5 
and  63  (p.  125).  Which  of  these,  if  either, 
represents  the  weight  of  the  atom  ?  The 
specific  heat  of  copper  is  -994,  and  6-4  -f-  -994 
=  64-4.  There  is  no  doubt,  therefore,  in 
spite  of  the  deviation,  that  the  second  of  these 
equivalents  represents  the  weight  of  the  atom. 
Atomic  Aggregates  in  Solution. — The  re- 
sources at  the  disposal  of  chemists  for  deter 
mining  the  molecular  weights  of  compounds 
which  cannot  be  gasified  have  been  added  to 
in  recent  years  by  the  introduction  of  other 
auxiliary  methods.  These  newer  methods 
are  based  upon  physical  principles,  which 
cannot  be  considered  in  detail  here ;  but  a 
general  notion  can  be  outlined.  Let  us  begin 


AUXILIARY   METHODS          177 

with  some  facts.  Everybody  is  familiar 
with  the  fact  that  certain  liquids,  such  as 
water,  dissolve  certain  solids  (p.  85),  such  as 
sugar  or  salt.  Other  liquids,  such  as  alcohol 
(spirit  of  wine),  will  mix  with,  i.e.,  dissolve  in 
water  in  all  proportions.  So,  also,  gases 
dissolve  in  liquids  like  water,  and  such  solu- 
tions give  up  the  gas  again  when  heated. 
This  property  of  liquids  may  be  looked  upon 
as  a  physico-chemical  property,  because  it  is 
specific ;  solubility  and  insolubility,  or  the 
degree  of  solubility  under  given  conditions, 
are  properties  dependent  upon  the  nature  of 
the  solvent  and  solute.  By  "  nature  "  in  this 
case,  chemical  nature  is  meant.  For  the 
present  purpose  water  may  be  taken  as  a 
typical  solvent,  and  sugar  as  a  typical  solute. 
This  familiar  substance  is  composed  of  the 
three  elements,  carbon,  hydrogen,  and  oxygen, 
in  the  atomic  proportions  indicated  by  the 
formula  C12H22On.  This  is  the  result  of  the 
analysis  of  sugar,  which  contains  in  100  parts 
by  weight  42-1  parts  of  carbon,  6-4  of  hydro- 
gen, and  51-5  of  oxygen.  The  formula,  as 
in  all  such  cases,  is  simply  the  percentage 
composition  translated  into  terms  of  atomic 
weights  (in  round  numbers,  C  =  12  ;  H  =  1  ; 
O  =  16).  But  this  formula  is  a  minimum 
formula  ;  the  sum  total  of  the  atomic  weights 


178  CHEMISTRY 

will  be  found  to  be  342,  which  may  or  may 
not  be  the  molecular  weight.  The  molecule 
cannot  weigh  less  than  this,  because  if  we 
halve  it  we  introduce  the  inadmissible  formula, 
C6HnO5i,  containing  a  half  atom  of  oxygen. 
Neither  can  it  be  decided  whether  the  mole- 
cular weight  is  a  multiple  of  342  by  determin- 
ing the  vapour  density,  because  sugar  is 
completely  decomposed  by  heat.  This  illus- 
trates a  class  of  cases  in  which  the  newer 
methods  supply  the  required  information. 

The  point  on  a  thermometer  scale  at  which 
a  liquid  when  cooled  passes  into  the  solid 
state  is  said  to  be  the  freezing-point ;  and  the 
point  at  which  the  liquid  when  heated  passes 
into  vapour  under  any  given  pressure  is  the 
boiling-point  at  that  pressure.  Thus,  the 
freezing-point  of  water  is  0°  (273°  absolute), 
and,  under  ordinary  atmospheric  pressure, 
the  boiling-point  is  100°  (373°  absolute). 
Now,  it  is  a  well  known  fact  that  a  solution 
freezes  at  a  lower  temperature,  and  boils  at 
a  higher  temperature,  than  the  pure  solvent. 
The  physical  interpretation  of  this  fact  cannot 
be  discussed  here  ;  the  two  properties  referred 
to  are  really  different  aspects  of  the  same 
physical  principle,  and  so  the  freezing-point 
only  may  be  considered  in  order  to  simplify 
matters. 


AUXILIARY   METHODS          179 

It  has  been  established  by  experiment 
(Raoult,  1883)  that  there  is  a  definite  relation- 
ship between  the  depression  of  freezing-point 
of  a  solution  and  the  molecular  weight  of 
the  solute.  That  relationship  is  broadly  ex- 
pressed by  the  statement  that,  for  any  particu- 
lar solvent  and  with  equal  concentration  (i.e., 
percentage  of  solute  in  solvent),  weights  of 
different  substances  corresponding  to  the 
molecular  weights  produce  the  same  amount 
of  depression.  It  is  not  possible  to  enter 
further  into  details  either  of  principle  or  of 
technique,  but  it  will  be  seen  in  a  general  way 
that  the  introduction  of  this  principle  adds  to 
the  methods  available  for  the  determination 
of  molecular  weights.  If  the  depression 
produced  by  any  particular  solvent  with 
various  solutes  of  known  molecular  weight 
(i.e.,  determined  by  other  methods)  in  known 
degrees  of  concentration  is  ascertained  experi- 
mentally— and  the  more  dilute  the  solution 
the  more  concordant  the  results — then  the 
molecular  weight  of  a  substance  of  unknown 
molecular  weight  can  be  determined  by  a 
simple  calculation  when  the  depression  of 
freezing-point  produced  by  a  known  weight  of 
the  substance  in  a  known  weight  of  the  solvent 
is  ascertained  by  observation. 

It  must  be  understood  that  this  method  is 


180  CHEMISTRY 

applicable  only  in  cases  where  there  is  no 
chemical  action  between  the  solvent  and  solute. 
It  must  also  be  pointed  out  that  the  method 
is  inapplicable  to  acids,  bases,  or  salts — in 
other  words,  to  electrolytes  (p.  109) — because 
there  is  evidence  that  in  the  case  of  these 
compounds  solution,  at  any  rate  in  water,  is 
accompanied  by  a  resolution  of  the  com- 
pound into  component  parts  corresponding 
with  those  which  travel  to  the  respective 
poles  during  the  process  of  electrolysis  (p.  111). 
In  such  solutions  there  are,  therefore,  con- 
tained a  larger  number  of  component  parts — 
known  as  ions — than  is  the  case  with  un- 
resolvable  non-electrolytes,  such  as  sugar ; 
and  the  depression  of  freezing-point  produced 
by  compounds  which  undergo  this  ionic 
dissociation  is,  therefore,  incomparable  with 
that  produced  by  compounds  of  the  other 
type. 

From  the  practical  side,  the  reader  must 
realize,  also,  that  the  depression  which  is 
measured  in  these  cases  is  extremely  small, 
and  necessitates  a  refinement  in  thermometric 
methods  quite  beyond  the  ordinary  experience 
of  the  casual  observer.  To  give  one  illustra- 
tion, it  may  be  mentioned  that  a  solution  of 
one  part  of  sugar  in  100  parts  of  water  pro- 
duces a  depression  of  -058°,  i.e.,  the  solution 


AUXILIARY   METHODS          181 

solidifies  at  —  0-058°  instead  of  at  0°.  The 
depressing  value  of  water  being  known  for 
many  compounds  of  known  molecular  weight, 
it  will  be  found  on  calculation  that  the  de- 
pression produced  by  sugar  corresponds  most 
closely  with  the  molecular  weight  342,  and 
that  the  formula  C12H22O11  is,  therefore,  the 
molecular  formula.  This  means  that  the 
molecule  of  sugar  contains  12+22+11  =  45 
atoms,  and  that  its  weight  on  the  hydrogen 
scale  is  342.  The  state  of  atomic  aggregation 
in  which  sugar  exists  in  solution  is  the  same 
as  that  which  it  would  possess  in  the  state  of 
vapour,  were  it  possible  to  convert  this  com- 
pound into  vapour  without  decomposing  it. 

The  contemplation  of  such  conclusions  as 
have  now  been  set  forth — conclusions  based 
upon  experimental  observations  interpreted 
by  hypothesis — will  assuredly  serve  to  justify 
the  claim  of  Chemistry  to  take  rank  as  a 
science  which  is  penetrating  more  and  more 
deeply  into  the  inner  mysteries  of  matter. 
The  atoms  and  molecules  which  are  dealt 
with  by  such  methods  as  have  been  considered 
— methods  which  are  by  no  means  exhaustive 
of  all  our  resources — are  legitimately  con- 
ceived as  physical  entities  ;  and  every  advance 
in  our  knowledge  of  the  physical  and  chemical 
properties  of  matter  has  served  to  strengthen 


182  CHEMISTRY 

the  reality  of  this  conception.  Some  further 
developments  will  be  discussed  as  far  as 
possible  in  the  short  space  remaining  at  our 
disposal.  It  may  be  fairly  asserted  that  the 
atoms  and  molecules  which  modern  chemical 
philosophy  has  called  into  existence  "  out  of 
the  void  and  formless  infinite  "  have  become 
the  common  property  of  thinkers  and  workers 
in  every  department  of  science. 


CHAPTER   VIII 

DETERMINATION  OF  THE  RELATIVE  WEIGHTS 
OF  THE  ATOMS — THE  ISOLATION  OF  DE- 
FINITE SUBSTANCES CHEMISTRY  AS  AN 

EXACT  SCIENCE — THE  STANDARD  OF 
ATOMIC  WEIGHTS — CHEMICAL  ARITHMETIC 
— VOLUMETRIC  RELATIONSHIPS 

Determination  of  the  Relative  Weights  of 
Atoms. — The  fundamental  units  of  matter 
which  have  thus  been  made  the  basis  of  the 
modern  interpretation  of  chemical  phenomena 
exist  in  the  82  different  modifications  re- 
presenting the  known  chemical  elements. 
The  varying  characters  of  the  individual 
elements  are  qualitative  expressions  of  the 
idiosyncrasies  of  the  atoms ;  with  the  pro- 
gress of  discovery,  we  may  hope  to  be  able 
to  express  these  differences  more  and  more 
precisely  in  quantitative  terms.  It  has  been 
shown  in  the  previous  chapters  that  the  main 
attribute  of  the  atom  which  is  at  present  dealt 
with  quantitatively  is  the  relative  weight. 
The  question  now  arises — how  is  this  weight 

183 


184  CHEMISTRY 

ascertained  ?  The  relative  weight  of  an 
atom  is  a  "  Constant  of  Nature  "  in  the  same 
sense  as,  let  us  say,  the  length  of  an  ethereal 
wave  corresponding  to  a  particular  colour, 
or  the  number  of  vibrations  corresponding  to 
a  particular  musical  note.  It  is  evident, 
therefore,  that  a  character  of  such  fundamental 
importance  as  the  atomic  weight  must  be 
expressed  numerically  with  the  utmost  obtain- 
able accuracy. 

In  the  foregoing  chapters,  certain  atomic 
weights  have  already  been  assigned  to  some 
of  the  elements.  The  reader  has  been  given 
to  understand  in  a  general  way  that  these 
numbers  are  found  by  determining  the  relative 
quantities  of  the  elements  present  in  com- 
pounds, and  then  ascertaining  the  molecular 
weight  of  the  compound  by  one  or  another  of 
the  available  methods.  It  must  be  realized 
that  the  physical  methods,  such  as  vapour 
density  determination,  depression  of  freezing- 
point,  raising  of  boiling-point,  determination 
of  specific  heat,  etc.,  are  not  methods  of 
precision  in  the  strict  sense,  but  methods  of 
control — they  decide  only  the  total  number 
of  atoms  composing  a  molecule,  and  the 
particular  equivalent  or  combining  weight 
which  represents  the  weight  of  the  atom. 
If,  therefore,  these  methods  are  methods  of 


ATOMIC    WEIGHTS  185 

control,  what  is  it  that  they  do  control  ? — 
evidently  the  equivalents  determined  by 
analysis  or  synthesis. 

The  determination  of  the  atomic  weight  of 
an  element  resolves  itself  into  a  question  of 
quantitative  composition  as  ascertained  with 
all  the  precision  attainable  by  human  skill.  It 
is  not  a  question  as  to  whether  the  atomic 
weight  of  oxygen  is  8  or  16,  of  sulphur  16  or 
32,  of  carbon  3  or  12,  of  nitrogen  4|  or  14,  of 
iron  28  or  56,  of  mercury  100  or  200 — these 
broad  issues  may  be  taken  as  settled  by  the 
methods  of  control.  It  is  now  a  question  of 
accuracy  in  the  decimal  places,  for  these  atoms 
are  the  gifts  of  Chemistry  to  universal  science, 
and  the  account  of  their  attributes  must  be 
rendered  with  all  the  precision  demanded  by 
the  exact  sciences. 

It  is  beyond  the  scope  of  this  work  to 
describe  in  detail  the  methods  of  determining 
atomic  weights.  The  chemical  changes  dealt 
with  are  for  the  most  part  simple.  For 
example,  direct  combination  between  a  metal 
and  a  halogen  when  giving  rise  to  a  definite 
weighable  compound  gives  the  ratio  : — Metal : 
Halogen.  As  an  actual  case,  the  determination 
of  the  equivalent  of  silver  with  respect  to 
chlorine  may  be  cited.  A  known  weight  of 
silver  converted  into  chloride  by  combination 


186  CHEMISTRY 

with  chlorine  gives  so  much  silver  chloride. 
Using  symbols,  and  expressing  the  change  in 
the  form  of  an  equation,  we  have  : — 2  Ag  + 
C12  =  2  Ag  Cl.  The  gain  in  weight  represents 
the  chlorine  combined  with  the  silver ;  the 
ratio  Ag  :  Cl  has  been  found.  It  will  be  noted 
incidentally  that  no  molecular  formula  has 
been  assigned  to  the  silver,  because  the  state 
of  atomic  aggregation  of  the  solid  metal  is 
unknown  (p.  172) :  it  may  be  Agx,  the  value  of 
x  being  unknown. 

Again,  most  of  the  elements  can  be  made 
to  combine  either  directly  or  indirectly  with 
oxygen.  When  the  oxide  is  a  definite  com- 
pound, the  weight  of  oxide  obtained  from  a 
known  weight  of  the  element  gives  the  ratio  : 
Element  :  Oxygen.  Conversely,  an  oxide  of  de- 
finite composition  when  resolvable,  say  by 
heat,  into  element  and  oxygen,  gives  the  same 
information.  Many  metallic  oxides  which  do 
not  part  with  their  oxygen  on  heating  alone 
are  reduced  to  the  metallic  state  when  heated 
with  some  element  which  can  combine  with 
the  oxygen.  Carbon  and  hydrogen  are  such 
reducing  agents.  The  oxides  of  iron,  for 
example,  can  all  be  reduced  to  iron  by  strongly 
heating  them  with  charcoal  or  some  other 
form  of  carbon.  That  is  why  the  metal  iron 
was  referred  to  as  an  artificial  product 


ATOMIC    WEIGHTS  187 

(p.  87),  because  the  metal  is  obtained  from  its 
ores  by  such  treatment.  The  oxides  can  also 
be  reduced  by  heating  them  in  an  atmosphere 
of  hydrogen,  and  this  completes  the  proof  that 
these  oxides  consist  only  of  metal  and  oxygen 
(p.  71).  The  loss  of  weight  thus  undergone  by 
the  oxide  on  reduction  by  hydrogen  might  be 
made  to  give  the  ratio  : — Metal :  Oxygen.  And, 
since  in  this  case  the  oxygen  of  the  oxide  forms 
water  with  the  hydrogen,  the  weight  of  water 
gives  also  the  ratio  O  :  H  in  water.  To  take 
another  example : — The  oxide  of  copper  formed 
when  copper  is  heated  in  oxygen  (p.  84)  is  a 
perfectly  definite  oxide,  which  is  reduced  to 
copper  on  heating  with  carbon  or  in  hydrogen, 
the  change,  under  the  latter  condition,  being 
CuO  +  H2  =  Cu  +  H2O.  Here  again  it  will  be 
noted  that  no  molecular  formula  is  assigned 
to  the  oxide  or  to  the  metal,  the  reason  being 
as  above  given — our  ignorance  of  the  mole- 
cular weights  of  solid  elements  and  com- 
pounds. In  this  example,  a  known  weight  of 
oxide  loses  so  much  on  reduction ;  the  loss 
represents  oxygen.  This  known  weight  of 
oxygen  gives  so  much  water,  the  gain  in 
weight  being  due  to  the  hydrogen  combined 
with  that  quantity  of  oxygen — the  ratio  O  :  H 
in  water  has  been  determined. 

There  is  no  royal  road  to  the  determination 


188  CHEMISTRY 

of  atomic  weights — every  available  method  is 
utilized  :  it  is  entirely  a  question  of  practica- 
bility. Since,  in  the  case  of  some  of  the 
elementary  gases,  the  densities  referred  to 
hydrogen  represent  the  atomic  weights  (p. 
166),  a  direct  determination  of  density  gives 
the  necessary  information  with  a  degree  of 
precision  limited  only  by  the  accuracy  of  the 
experimental  methods.  If  a  metal  can  be 
deposited  in  a  weighable  condition  by  the 
electrolytic  decomposition  of  its  salts,  the 
electro-chemical  equivalent  (p.  121),  gives  the 
required  information.  It  can  be  asserted  as  a 
general  principle  that  for  the  determination  of 
atomic  weights  the  main  requirement  is 
purity — the  materials  used,  elements  and 
compounds,  must  be  chemical  individual 
substances  in  the  strictest  sense  realizable. 

The  Isolation  of  Definite  Substances. — The 
whole  development  of  Chemistry  is  intimately 
bound  up  with  the  practical  necessity  of 
isolating  the  various  forms  of  matter,  element- 
ary and  compound,  in  such  a  state  of  purity 
that  a  chemical  individual  substance  is 
obtained.  The  foundations  of  our  science  are 
based  on  the  study  of  individual  substances  ; 
and  the  degree  of  exactness  that  has  been 
reached  is  a  measure  of  the  success  of  our 
laboratory  methods.  The  reader  who  ap- 


ATOMIC    WEIGHTS  189 

preaches  Chemistry  from  the  purely  literary 
side  must  thoroughly  grasp  this  fundamental 
reality — he  must  realize  to  the  full  extent  the 
significance  of  the  statement  that  Chemistry 
is  an  art  as  well  as  a  science  (p.  31).  In  no 
branch  of  practical  work  is  the  standard  of 
individuality,  i.e.,  of  purity,  of  more  vital 
importance  than  in  the  materials  used  for  the 
determination  of  atomic  weights.  It  is  for 
this  reason  that  the  general  question  of  the 
isolation  of  individual  substances  has  been 
brought  forward  here.  The  discussion  of 
practical  methods  is  beyond  the  limits  of  this 
work ;  they  cannot  be  mastered  by  simply 
reading  about  them,  but  only  by  that  com- 
bination of  manual  skill,  judgment,  and 
resourcefulness  which  is  essential  for  accuracy 
in  this  kind  of  work.  The  separation  of 
individual  substances — in  other  words,  their 
purification,  is  effected  by  processes  which 
are  familiar  enough  as  laboratory  operations. 
If  a  substance  has  a  definite  boiling-point 
(p.  59),  it  can  be  distilled,  and  a  fraction 
having  a  constant  boiling-point  isolated  ;  if 
it  can  be  vaporized  by  heat  and  condensed  in 
the  solid  form,  it  can  be  purified  by  sublima- 
tion ;  if  it  separates  from  its  solution  in  a 
crystalline  form  when  the  solvent  is  evapor- 
ated, the  associated  impurities  can  in  this 


190  CHEMISTRY 

way  be  removed  by  a  sufficient  number  of 
crystallizations  ;  if  the  substance  in  solution 
can  by  interaction  with  some  other  substance 
be  converted  into  an  insoluble  compound  it 
can  be  purified  by  precipitation. 

But,  in  spite  of  all  our  resources,  an  abso 
lutely  pure  substance  is  so  extremely  difficult 
to  obtain  that  it  may  almost  be  regarded 
as  a  mathematical  abstraction.  Matter  pure 
enough  to  withstand  chemical  examination 
may  still  be  shown  by  more  delicate  physical 
tests,  such  as  by  means  of  the  spectroscope,  to 
contain  traces  of  foreign  substances.  The 
atomic  weights  in  use  are  necessarily  of  different 
degrees  of  exactness,  and  finality  has  not  yet 
been  reached — the  work  is  still  in  progress, 
and  accuracy  is  being  pushed  further  along  the 
line  of  decimal  places.  The  results  obtained 
by  different  experimenters  are  considered  by 
an  International  Commission,  under  whose 
auspices  a  list  of  atomic  weights  is  published 
annually.  The  list  for  1912  is  given  at  the  end 
of  this  volume. 

Chemistry  as  an  Exact  Science. — It  is  not 
claimed  that  Chemistry  ranks  with  the  exact 
sciences  in  the  sense  of  having  reached  the 
purely  deductive  stage  ;  but  in  every  direction 
in  which  quantitative  treatment  is  possible 
the  relative  weights  of  the  atoms  come  sooner 


ATOMIC    WEIGHTS  191 

or  later  into  consideration.  Hence  the  neces- 
sity for  accuracy  in  these  constants,  the 
responsibility  for  the  determination  of  which 
is  necessarily  thrown  ultimately  upon  the 
chemical  balance.  This  instrument,  com- 
pared with  those  used  in  some  of  the  most 
delicate  physical  measurements,  may  perhaps 
be  considered  coarse.  But,  for  all  practical 
purposes,  accuracy  to  the  l-10,000th  of  a 
gram,  i.e.,  l-10th  of  a  milligram,  is  sufficient; 
for  the  more  refined  work  sensitiveness  to 
1 -200th  of  a  milligram  is  obtainable.  The  time 
may  come  when,  for  the  investigation  of  the 
more  recondite  attributes  of  the  atom,  a 
higher  degree  of  precision  will  be  necessary  ; 
and  it  may  be  well  to  point  out,  therefore,  that 
there  has  recently  been  added  to  the  resources 
of  the  physicist  and  chemist  a  micro-balance 
constructed  of  quartz  capable  of  weighing 
the  almost  inconceivably  small  quantity  of 
1-10, 000th  of  a  milligram. 

But  the  difficulty  now  rests  not  so  much 
with  the  instrument  as  with  the  matter  ; 
ultra  refinement  in  weighing  unless  the  sub- 
stance is  really  "  individual  "  is  suggestive  of 
"  straining  at  a  gnat  and  swallowing  a  camel." 
Some  of  the  atomic  weights  are  confessedly 
uncertain  ;  on  account  of  those  difficulties 
of  purification  which  have  just  been  indicated, 


192  CHEMISTRY 

it  is  recognized  that  in  many  cases  revision  is 
necessary.  In  no  case,  however,  is  the 
responsibility  of  fixing  the  atomic  weight 
thrown  upon  the  analysis  or  synthesis  of  one 
compound  of  an  element  when  several  com- 
pounds are  available.  The  degree  of  precision 
reached  is  evidently  dependent  upon  the  num- 
ber of  independent  sets  of  observations  ;  all 
those  atomic  weights  which  have  been  deter- 
mined with  the  greatest  accuracy  are  based 
upon  converging  lines  of  evidence.  In  modern 
times  the  science  of  Chemistry  largely  owes  its 
advance  towards  exactness  in  this  direction 
to  the  life-long  labours  of  men  like  Stas, 
Morley,  Richards  and  Clarke. 

The  Standard  of  Atomic  Weights. — Although 
hydrogen,  having  the  lowest  atomic  weight, 
was  at  first  naturally  taken  as  the  standard,  for 
practical  purposes  this  element  is  by  no  means 
a  convenient  one.  It  is  evident  that  any 
element  of  which  the  equivalent  with  respect  to 
hydrogen  has  been  determined  with  precision 
may  be  made  the  standard ;  the  translation  to 
the  hydrogen  scale  of  the  atomic  weight  deter- 
mined with  reference  to  such  other  element 
then  becomes  a  matter  of  calculation.  Hydro- 
gen forms  but  few  compounds  with  other 
elements  which  admit  of  satisfactory  manipula- 
tion for  quantitative  purposes.  Moreover,  the 


ATOMIC    WEIGHTS  193 

gas  itself  is  so  light  (0-0695  when  air  =  1)  that 
it  tends  to  diffuse  out  of  all  vessels,  and  is 
difficult  to  weigh,  so  that  the  direct  deter- 
mination of  its  atomic  weight  by  the  observa- 
tion of  its  density  (p.  166)  is  liable  to  error. 
Oxygen,  on  the  other  hand,  forms  compounds 
with  most  of  the  elements  ;  and,  in  fact,  many 
of  the  equivalents  which  have  been  deter- 
mined are  based  upon  the  analysis  of  oxygen 
compounds.  This  gas  has  also  the  advantage 
of  being  heavier  than  air  (1-105  when  air  =1), 
so  that  its  density  can  be  determined  with  less 
liability  to  error  than  hydrogen.  For  these 
and  other  reasons,  oxygen  is  now  made  the 
standard  of  atomic  weights ;  and  in  the 
international  list  the  numbers  adopted  are 
relative  to  that  element.  It  will  be  under- 
stood, therefore,  that  the  weight  of  the  atom 
of  oxygen  with  reference  to  hydrogen  is  a 
matter  of  extreme  importance.  The  ratio  of 
oxygen  to  hydrogen  in  water  obviously  furn- 
ishes the  necessary  data  ;  and  the  concentra- 
tion of  patience  and  skill  which  has  been 
brought  to  bear  upon  the  determination  of 
that  ratio  by  Morley  and  others  will  rank  in  the 
future  history  of  Chemistry  among  the  greatest 
achievements  in  scientific  precision. 

The   refined   experimental   methods   made 
use  of  in  fixing  this  ratio  cannot  be  described 


194  CHEMISTRY 

here.  The  equivalent  8  for  oxygen  when 
hydrogen  =  1  (p.  119)  must  be  corrected  in  the 
light  of  the  most  precise  evidence  to  7-94  ; 
and  so  the  atomic  weight  of  oxygen  on  this 
scale  becomes  15-88.  Or,  if  8  is  taken  as  the 
equivalent  of  oxygen,  then  the  atomic  weight 
of  hydrogen  becomes  1-008,  because  1  :  7-94 
=  1-008  :  8  (neglecting  the  last  decimals).  The 
molecular  weight  of  oxygen  is  thus  31-76  on 
the  scale  H=  1,  or  32  on  the  scale  H=  1-008 ; 
and  the  molecular  weight  of  hydrogen  is  then 
2-016.  These  results,  be  it  remembered,  are 
based  on  experimental  determinations  ;  and 
the  atomic  weights  will,  therefore,  differ 
according  to  the  standard  adopted,  but — 
and  here  the  whole  mystery  should  disappear 
— the  relative  weights  of  the  atoms  towards 
each  other  remain  unchanged,  and  that  is  all 
that  we  are  concerned  with  in  considering 
the  weight  of  an  atom  from  the  chemical 
point  of  view.  Consider  a  case.  On  the  scale 
H  =  1  and  O  =  15-88,  the  atomic  weight  of 
sulphur  is  31-83  ;  on  the  scale  H  =  1-008  and 
O  =  16,  it  is  32-07.  There  is  no  real  discrep- 
ancy here — it  is  simply  a  question  of  stand- 
ard ;  the  ratios  remain  the  same  : — neglecting 
last  decimals,  31-83  :  15-88  =  32-07  :  16,  or 
31-83  :  1  =  32-07  :  1-008  ;  and  so  with  refer- 
ence to  the  comparison  of  the  weight  of  the 


CHEMICAL    ARITHMETIC         195 

sulphur  atom  with  the  weight  of  any  other 
atom  on  the  two  scales.  The  two  sets  of 
atomic  weights  now  in  existence  should  thus 
be  easily  understood  ;  the  standard  O  =  16 
has  been  adopted  for  the  reasons  already 
stated.  The  atomic  weights  assigned  to  the 
elements  referred  to  in  the  previous  chapters 
have,  as  far  as  possible,  been  taken  as  whole 
numbers.  It  will  now  be  understood  that 
these  numbers  were  used  only  for  the  sake 
of  simplicity,  and  in  order  to  illustrate  general 
principles. 

Chemical  Arithmetic. — The  symbolical  lan- 
guage which  was  introduced  in  a  former 
chapter  (p.  137)  is  obviously  quantitative. 
The  formulae  assigned  to  atoms  and  molecules 
represent  so  much  by  weight  of  the  respective 
elements  or  compounds,  the  degree  of  numeri- 
cal precision  attaching  to  the  symbols  being 
the  degree  of  accuracy  with  which  the  relative 
weights  of  the  atoms  have  been  determined. 
It  will  be  seen,  therefore,  that  when  we  know 
the  formula  of  a  compound,  or  when  we  know 
the  composition  of  the  products  arising  from 
chemical  reaction  between  materials  of  known 
composition,  we  must  necessarily  bring  such 
symbols  into  the  domain  of  arithmetical 
treatment.  The  construction  of  a  formula 
from  the  results  of  analysis  is  in  itself  an 


196  CHEMISTRY 

arithmetical  problem.  For  example,  water 
has  been  said  to  contain  (in  round  numbers) 
oxygen  and  hydrogen  in  the  proportion  8  :  1 
— this  is  the  outcome  of  analysis  and  synthe- 
sis ;  and,  as  is  the  universal  custom,  the 
actual  numerical  results  are  in  the  first  place 
stated  on  the  percentage  scale ;  oxygen 
88-9,  and  hydrogen  11-1  per  cent.,  approxi- 
mately. That  gives  the  ratio  8:1.  But  for 
the  formula  we  want  not  the  weight  ratios 
only,  but  the  ratios  between  the  numbers  of 
atoms  in  the  molecule.  The  number  of  times 
the  atomic  weights  of  oxygen  and  hydrogen 
are  contained  in  these  percentage  numbers 
will  evidently  give  the  required  information. 
Thus  88-9  -M6  =  5-55,  and  11-1  -^  1  =  11-1, 
and  so  the  ratio  between  the  numbers  of  atoms 
is  5-55  : 11*1 ;  then,  since  the  atom  is  by 
hypothesis  indivisible,  we  take  the  nearest 
whole  numbers,  and  so  make  the  ratio  1 :  2. 
The  formula  is  thus  found  to  be  H2O  ;  and  if 
it  should  happen  to  be  a  multiple  of  this 
(which  in  this  case  it  is  in  the  liquid  state  ;  p. 
172),  it  makes  no  difference  to  the  relative 
numbers — the  minimum  formula  only  is 
found  by  this  calculation,  and  the  molecular 
formula  of  the  vapour  is  settled  by  the 
determination  of  the  molecular  weight  as 
already  explained. 


CHEMICAL   ARITHMETIC         197 

The  calculation  of  a  formula  from  the 
percentage  composition  is,  therefore,  simple 
enough ;  the  percentage  numbers  divided  by 
the  atomic  weights  and  the  results  expressed 
in  the  simplest  set  of  whole  numbers  give  the 
relative  numbers  of  the  respective  atoms  in 
the  molecule ;  the  actual  numbers  can  only 
be  found  when  the  molecular  weight  can  be 
determined.  Conversely,  the  formula  being 
known,  the  percentage  composition  can  be 
easily  calculated.  Sugar,  for  example,  is 
found  by  analysis  to  contain  carbon,  hydro- 
gen, and  oxygen  in  such  proportions  as  to 
lead  to  the  formula  C12H22On  (p.  177).  The 
sum  total  of  the  weights  of  the  atoms  being 
in  round  numbers  342,  that  weight  contains 
144  parts  of  carbon,  22  parts  of  hydrogen 
and  176  parts  of  oxygen,  from  which  fact 
the  weights  of  carbon,  hydrogen  and  oxygen 
contained  in  100  parts  of  sugar  can  obviously 
be  calculated  by  a  very  familiar  process. 

It  is  unnecessary  here  to  set  problems  or  to 
work  out  examples ;  we  are  concerned  only 
with  those  broad  principles  which  serve  to 
illustrate  the  enormous  power  which  the 
atomic  theory  has  placed  in  the  hands  of  the 
chemist.  An  obvious  extension  of  the  fore- 
going principles  enables  us,  for  instance,  to 
calculate  the  theoretical  yields  of  products 


198  CHEMISTRY 

resulting  from  chemical  change.  Such  changes 
— chemical  reactions — are  capable  of  being 
represented  in  the  form  of  equations  showing 
the  arrangement  of  the  materials  before  and 
after  the  reaction  (pp.  169  and  187).  Consider, 
by  way  of  illustration,  the  burning  of  iron  in 
oxygen  to  form  scale  (p.  23),  the  decomposition 
of  water  by  heated  iron  to  form  this  same 
oxide  (p.  81),  or  the  reduction  of  copper 
oxide  to  copper  by  means  of  hydrogen  just 
referred  to  (p.  187).  The  equations  are  : — 

3Fe+202  =  Fe304; 

3  Fe+  4  H2O  =  Fe3O4  +  4  H2 

CuO  +  H2  =  H2O  +  Cu. 

It  is  obvious  that,  given  the  atomic  weights, 
all  these  reactions  can  be  dealt  with  arith- 
metically. Using  the  nearest  whole  numbers, 

3  x  56  parts  of  iron  yield  3  x  56  +  16  x  4  parts 
of  magnetic  oxide  ;  3  x  56  parts  of  iron  use  up 

4  x  (16+  2)  parts  of  water  to  form  the  weight 
of  oxide  represented  by  Fe3O4,  and  liberate  8 
parts  of  hydrogen  ;  the  weight  of  copper  oxide 
represented  by  CuO  furnishes  the  weight  of 
copper  represented  by  the  atomic  weight,  i.e., 
63-6  when  O  =  16,  or  63-1  when  H  =  1.     From 
such  data  it  is,  of  course,  easy  to  find  out  how 
much  magnetic  oxide  can  be  obtained  from 
any  given  weight  of  iron,  how  much  hydrogen 


CHEMICAL    ARITHMETIC         199 

a  known  weight  of  iron  will  yield,  or  how 
much  copper  and  how  much  water  can  be 
obtained  from  a  certain  quantity  of  copper 
oxide. 

It  is  needless  to  enlarge  upon  this  topic, 
but  the  practical  bearing  of  theory  upon 
industrial  processes  finds  no  better  illustration 
in  the  whole  domain  of  applied  science  than 
in  this  power  of  prevision  which  the  chemist 
is  thus  enabled  to  wield.  From  known 
weights  of  materials  reacting  in  a  known 
way  and  giving  rise  to  known  products,  it  is 
possible  to  calculate  what  the  yields  should  be. 
Not  that  theoretical  yields  are  often  actually 
realized  in  practice — there  are  generally 
secondary  changes,  unavoidable  losses,  and 
so  forth  ;  but  the  nearer  the  theoretical  yield 
the  more  successful  the  industry — the  trans- 
formation of  industrial  empiricism  into  scienti- 
fic procedure  is  measured  quantitatively 
by  the  percentage  of  the  theoretical  yield. 
It  is  only  in  the  most  refined  laboratory 
operations  that  the  requirements  of  theory 
and  the  results  of  experiment  coincide. 

Volumetric  Relationships. — The  volumes 
occupied  by  known  weights  of  gaseous  com- 
pounds being  known,  it  is  evident  that 
here  also  arithmetical  treatment  is  possible. 
Facts  and  arguments  have  already  made  it 


200  CHEMISTRY 

clear  to  the  reader  that  in  the  gaseous  state 
the  molecular  weights  of  all  non-dissociable 
elements  and  compounds  occupy  the  same 
volumes.  This  is  the  obverse  side  of  the 
hypothesis  of  Avogadro  (p.  157).  In  order 
to  simplify  matters,  we  can,  therefore,  take 
as  reference  unit  some  standard  element  of 
which  the  volume  of  a  given  weight  is  known 
with  accuracy.  In  fact,  the  molecule  of 
hydrogen  in  grams,  2  x  1-008  =  2-016  (p.  194), 
occupies  22-4  litres  at  the  standard  tempera- 
ture and  pressure  of  0°  and  760  mm.  The 
gram-molecule  of  every  non-dissociable  element 
and  compound,  therefore,  under  these  com- 
parable conditions  fills  a  space  of  22-4  litres. 
The  rest  is  simple  arithmetic.  A  given  weight, 
say  of  iron,  yields  so  much  by  weight  of 
hydrogen  :  2-016  grams  of  hydrogen  at  0° 
and  760  mm.  occupy  22-4  litres  ;  therefore 
the  volume  of  hydrogen  liberated  by  the 
given  weight  of  iron  will  be  so  many  litres 
under  similar  conditions.  Again,  carbon 
(C  =  12)  when  heated  in  oxygen  burns,  i.e., 
undergoes  combustion  with  the  formation  of 
the  dioxide,  CO2  (p.  125).  The  reaction  is 
C  -|-  Oa  =  CO2,  whence  12  grams  of  carbon 
give  12  +(16  x  2)  =44  grams  of  the  dioxide. 
That  weight  occupies  the  same  volume  as  the 
molecule  of  hydrogen  under  comparable  con- 


CHEMICAL   ARITHMETIC         201 

ditions — it  is  the  gram-molecule  of  carbon 
dioxide ;  so  that  if  12  grams  of  carbon  give 
22-4  litres  of  the  dioxide  under  the  conditions 
specified,  the  volume  of  gas  given  by  any 
specified  weight  of  carbon  can  be  calculated. 

From  the  conception  of  equimolecular 
weights,  and  therefore  of  equal  numbers  of 
molecules  of  elements  and  compounds  occupy- 
ing equal  volumes  under  comparable  condi- 
tions, it  is  but  a  natural  step  to  the  conception 
of  solutions  containing  equal  numbers  of 
molecules  in  equal  volumes.  Such  solutions 
can,  of  course,  be  prepared.  The  molecular 
weight,  or  some  known  fraction  of  that  weight, 
in  grams  of  any  compound  AB  dissolved  in 
some  standard  volume  of  a  liquid,  say  water, 
contains  a  known  weight  of  AB  per  cubic 
centimetre.  The  standard  used  is  the  litre 
(1000  cubic  centimetres),  and  a  solution 
containing  the  gram  molecule  AB  per  litre  is 
called  a  normal  solution.  This  solution  can 
be  diluted  with  water  if  necessary,  and  made 
up  to  any  required  volume  so  as  to  be  semi- 
normal  (J  AB  per  litre),  deci-normal  (T\T  AB 
per  litre),  and  so  forth.  The  point  is  that 
such  solutions  contain  known  quantities  of 
AB  per  cubic  centimetre,  and  can  be  measured 
out  with  great  accuracy  by  suitable  apparatus. 

Supposing,  now,  that  the  compound  AB  is 


202  CHEMISTRY 

capable  of  reacting  with  another  compound 
CD  in  solution,  so  as  to  form  new  substances 
by  interchange  of  components,  then  we  have 
a  chemical  change  brought  about  by  what  is 
termed  double  decomposition.  Such  change 
can  be  expressed  by  the  usual  equation : 
AB  +  CD  =  AC  -f  BD,  and  in  this  form  can 
be  dealt  with  arithmetically.  It  will  be  seen 
that  we  have  in  this  case  quantities  of  two 
compounds  which  may  be  regarded  as  chemi- 
cally equivalent  (p.  121).  If,  therefore,  the 
precise  point  could  be  detected  at  which,  on 
adding  the  solution  of  AB,  the  whole  quantity 
of  CD  present  was  transformed  by  interaction 
with  AB  into  the  two  new  substances,  we 
should  be  enabled  to  determine  the  weight 
of  CD  in  a  solution  containing  an  unknown 
quantity  of  that  compound.  In  other  words, 
if  a  solution  of  AB  of  known  value  is  mixed 
continuously  with  the  solution  of  CD  and  the 
addition  arrested  at  the  precise  point  when  the 
equivalent  weight  of  AB  has  been  added, 
we  can  calculate  the  quantity  of  CD  present 
in  its  solution.  The  weight  of  AB  is  known 
from  the  measured  volume  delivered ;  the 
weight  of  CD  contained  in  a  measured  volume 
of  its  solution  is  at  first  unknown,  but  the 
weight  equivalent  to  so  many  grams  or  frac- 
tions of  grams  of  AB  is  settled  by  the  weight 


VOLUMETRIC   ANALYSIS        203 

of  AB  added.  So  many  cubic  centimetres  of 
the  solution  of  CD  required  so  many  cubic 
centimetres  of  the  solution  of  AB  to  hit  the 
point  of  equivalence — the  rest  is  a  matter  of 
arithmetic. 

All  these  conditions  can  be  realized  ex- 
perimentally. There  are  large  numbers  of 
chemical  reactions  of  this  type  which  are  so 
sharp  that  the  exact  point  when  equivalence 
between  the  reacting  materials  has  been  struck 
can  be  indicated  by  the  change  of  colour  of 
some  added  substance  which  is  not  chemically 
concerned  in  the  reaction  (known  as  an 
indicator),  or  by  other  means.  On  this 
principle — which  is  necessarily  outlined  here 
only  in  a  very  broad  way — there  is  founded 
a  largely  used  and  beautifully  delicate  method 
of  determining  with  very  great  precision  the 
actual  weights  of  substances  contained  in 
solutions.  It  is  the  method  appropriately 
known  as  Volumetric  Analysis. 


CHAPTER   IX 

VALENCY — CHEMICAL   STRUCTURE — THE 
CHEMISTRY   OF   CARBON — STEREO- 
CHEMISTRY 

Valency. — In  its  simplest  aspect,  chemical 
combination  is,  from  the  point  of  view  now 
reached,  union  between  elementary  atoms. 
The  equivalent  being  that  weight  of  an  element 
which  combines  with  or  replaces  one  part  by 
weight,  i.e.,  one  atom  of  hydrogen  (p.  119), 
it  follows  that  the  number  of  times  the 
equivalent  is  contained  in  the  atomic  weight 
of  any  particular  element  will  represent  the 
capacity  of  that  particular  atom  for  combining 
with  hydrogen.  If  the  element  does  not  form 
compounds  with  hydrogen,  then  the  number 
expresses  the  combining  capacity  of  the  atom 
for  some  other  atom  which  is  equivalent  to 
the  hydrogen  atom.  The  principle  is  a 
familiar  one — atoms  which  are  equivalent 
to  the  same  atom  (hydrogen)  are  equivalent 
to  one  another.  A  new  property  of  the 
chemical  atom  is  thus  brought  out,  viz.,  its 
204 


VALENCY  205 

value  as  measured  by  the  number  of  atoms 
of  hydrogen  or  equivalent  atoms  with  which  it 
can  combine.  This  property  is  appropriately 
described  as  the  valency  of  the  atom.  If  the 
atomic  weight  contains  the  equivalent  once, 
i.e.,  if  the  equivalent  and  atomic  weight 
are  identical,  that  atom  can  combine  only 
with  one  atom  of  hydrogen  or  of  chlorine, 
bromine,  etc.  The  formulae  of  the  com- 
pounds HC1,  HBr,  etc.  (p.  165),  express  this 
fact.  If  the  equivalent  is  contained  twice 
in  the  atomic  weight,  then  that  atom  can 
obviously  combine  with  two  atoms  of  hydro- 
gen, chlorine,  etc.  ;  if  it  is  contained  three 
times  in  the  atomic  weight,  the  combining 
capacity  or  valency  of  the  atom  is  three,  and 
so  forth. 

In  the  light  of  this  principle,  consider  some 
of  the  compounds  which  have  already  been 
made  use  of  for  the  purpose  of  illustrating 
other  principles.  The  atomic  weight  of  oxy- 
gen (16)  contains  two  equivalents  (8) ;  the 
atom  of  oxygen  can  therefore  combine  with 
two  atoms  of  hydrogen,  as  in  water,  H2O. 
The  atomic  weight  of  nitrogen  (taken  as  14) 
contains  the  equivalent  (4|)  three  times,  so 
that  the  nitrogen  atom  combines  with  three 
atoms  of  hydrogen,  as  in  ammonia,  NH3. 
Again,  the  atomic  weight  of  carbon  (taken  as 


206  CHEMISTRY 

12)  contains  the  lowest  equivalent  (3)  four 
times ;  the  simplest  known  compound  of 
carbon  with  hydrogen  is  methane,  or  marsh 
gas,  CH4,  the  gas  which  is  formed  by  the 
decay  of  vegetable  matter  under  water,  and 
which  rises  in  bubbles  to  the  surface  when  the 
mud  at  the  bottom  of  a  stagnant  pond  is 
stirred  up.  Moreover,  since  one  atom  of  car- 
bon can  combine  with  or  be  saturated  by  four 
atoms  of  hydrogen,  and  since  two  atoms  of 
hydrogen  combine  with  one  atom  of  oxygen, 
two  atoms  of  oxygen  are  equivalent  in  com- 
bining value  to  four  atoms  of  hydrogen  ;  and 
so  the  oxide  of  carbon  referred  to  as  the  product 
of  the  burning  of  carbon  in  oxygen  (p.  125)  is 
the  dioxide,  CO2. 

The  broad  principle  of  valency  should  now 
become  clear  without  further  illustration. 
It  will  be  seen  that  we  have  in  this  doctrine  a 
means  of  classifying  the  atoms  irrespective 
of  their  nature,  i.e.,  of  the  kind  of  elementary 
matter  built  up  of  such  atoms.  Hydrogen, 
chlorine,  etc.,  are  univalent  elements,  oxygen 
bivalent,  nitrogen  tervalent,  carbon  quadri- 
valent, and  so  forth.  It  will  be  seen,  also, 
that  the  valency  is  not  always  a  fixed  number, 
but,  since  an  element  can  have  more  than  one 
equivalent,  in  some  cases  the  atom  must 
have  more  than  one  valency.  Carbon,  for 


VALENCY  207 

example,  is  generally  quadrivalent,  but  in 
some  of  its  compounds  it  may  be  bivalent ; 
iron  in  the  oxide  FeO  and  in  the  iodide, 
FeI2  (p.  38)  is  bivalent,  in  the  oxide  of  rust, 
Fe2O3,  it  may  be  tervalent  or  quadrivalent. 
Sulphur  is  quadrivalent  in  the  dioxide,  which 
is  formed  when  the  element  burns  in  air, 
SO2  (p.  128) ;  the  latter  can  be  made  to  com- 
bine with  another  atom  of  oxygen  to  form  a 
trioxide,  SO3,  (p.  128),  in  which  the  sulphur 
atom,  being  combined  with  3  bivalent  oxygen 
atoms,  is  itself  sexavalent.  Moreover,  sulphur 
forms  a  gaseous  compound  with  hydrogen, 
hydrogen  sulphide  or  sulphuretted  hydrogen, 
in  which  one  atom  of  sulphur  is  combined 
with  two  atoms  of  hydrogen,  SH2 ;  the 
atom  is  bivalent. 

The  conception  of  valency  may,  perhaps, 
on  these  grounds,  be  regarded  as  somewhat 
vague  ;  nevertheless,  as  will  be  seen  immedi- 
ately, with  all  its  imperfections  it  has  played 
an  exceedingly  important  part  in  the  develop- 
ment of  modern  Chemistry.  It  corresponds 
with  some  underlying  reality ;  whether  it  is 
an  inherent  property  of  the  atom  as  an 
individual  particle,  or  the  result  of  the  inter- 
action of  reciprocal  forces  exerted  between 
combining  atoms,  cannot  at  present  be 
decided.  It  is  an  empirical  doctrine  as  it 


208  CHEMISTRY 

stands — it  is  descriptive  rather  than  ex- 
planatory ;  yet  it  describes  some  faculty 
potentially  or  actually  present  in  the  atomic 
mechanism.  It  serves  as  a  check  upon  chemi- 
cal formulation,  and  yet  it  fails  to  explain 
why  any  particular  formula  is  possible.  In 
other  words,  it  enables  us  to  assert  that,  when 
combination  does  take  place,  certain  rules — 
more  or  less  elastic — are  obeyed.  But  it  does 
not  tell  us  why  this,  that,  or  the  other  atom 
can  combine  with  certain  atoms  and  not 
with  others  ;  it  takes  no  account  of  selective 
idiosyncrasies  beyond  representing  the  facts 
which  are  the  expressions  of  these  idiosyn- 
crasies. The  fact  that  valency  is  associated 
with  certain  physical  properties  of  the  atom  is 
sufficient  indication  that  there  is  an  under- 
lying reality. 

Consider,  for  instance,  the  fact  that  the 
same  electric  current  causes  the  liberation  of 
equivalent  weights  of  chemical  elements  (p. 
121).  This  may  be  interpreted  as  an  ind  ication 
that  equivalent  weights  of  the  elements  are 
associated  with  equal  quantities  of  electricity 
— that  chemically  equivalent  weights  are  also 
electrically  equivalent.  One  step  further,  and 
we  arrive  at  the  conclusion  that,  if  an 
element  of  which  the  equivalent  and  atomic 
weights  are  identical  is  associated  with  on* 


VALENCY  209 

unit  of  electricity,  an  atom  of  which  the 
atomic  weight  is  n  times  the  equivalent  must 
carry  n  units  of  electricity.  In  other  words, 
a  hint  that  valency  may  be  connected  with, 
if  not  actually  expressive  of,  the  electric 
constitution  of  the  atom  is  hereby  suggested 
(Helmholtz,  1881). 

Again,  if  the  capacity  for  combination 
possessed  by  various  atoms  is  expressed  as 
a  geometrical  conception,  it  may  be  con- 
sidered that  the  space  required  for  the  group- 
ing of  the  attached  atoms  to  the  central 
attracting  atom  will  be  larger  in  proportion 
with  the  increase  of  valency  of  the  latter. 
The  "  sphere  of  influence  "of  a  bivalent 
atom  would  be  double  that  of  a  univalent 
atom,  of  a  quadrivalent  atom  four  times 
that  of  a  univalent  atom  and  so  forth.  Here, 
also,  there  is  correspondence  with  fact ;  the 
packing  in  space  of  the  atoms  which  build 
up  a  molecule,  finds  external  expression  in  the 
regular  crystalline  form  which  characterises 
the  majority  of  definite  chemical  compounds. 
There  is  correspondence  between  the  archi- 
tecture of  the  solid  crystal,  regarded  as  a 
structure  built  up  of  molecules,  and  the 
volume  occupied  by  these  molecules  regarded 
as  assemblages  of  atoms,  each  occupying  its 
own  "  sphere  of  influence "  (Barlow  and 


210  CHEMISTRY 

Pope,  1906).  The  conception  of  the  valency 
of  the  atom,  evolved  originally  from  the  study 
of  chemical  compounds  (Frankland,  1853 ; 
Kekule,  1860),  has  thus  been  extended  into 
the  domain  of  Crystallography,  that  branch 
of  science  which  is  concerned  with  the  study 
of  the  definite  geometrical  forms  assumed 
by  elements  and  compounds  under  appropriate 
conditions. 

Chemical  Structure. — The  actual  grouping 
of  the  atoms  composing  a  molecule  may  at 
first  sight  appear  to  be  a  problem  beyond 
human  ken.  Nevertheless  the  doctrine  of 
valency,  in  spite  of  its  incompleteness,  has 
enabled  chemists  to  make  enormous  advances 
in  this  direction.  The  study  of  chemical 
composition  and  decomposition — of  the  results 
of  dissecting  the  molecule  piecemeal  and, 
when  possible,  of  reconstructing  it  from  its 
component  atoms — renders  it  possible  to 
form  a  mental  picture  of  the  way  in  which 
the  atoms  are  grouped.  That  mental  picture 
was  a  blurred  and  imperfect  representation 
until,  at  the  touch  of  the  conception  of 
valency,  definiteness  was  substituted  for 
vagueness,  and  the  chemist  provided  with  a 
means  of  translating  his  crude  imagery  into 
manageable  formulae. 

This    development    is    made    possible    by 


CHEMICAL    STRUCTURE         211 

the  simple  device  of  joining  the  atoms  together 
—not  in  an  arbitrary  way,  as  has  been  done 
in  the  preceding  chapters,  but  in  the  order 
which  represents  the  actual  state  of  their 
combination.  Furthermore,  the  combination 
between  the  atoms  is,  so  to  say,  visualised  by 
giving  each  atom  its  valency,  and  by  imagin- 
ing that  the  lines  of  force  which  bind  the 
atoms  are  real  lines.  These  "  bonds  "  are,  of 
course,  imaginary  ;  they  have  no  more  real 
existence  than  the  "  lines  of  force  "  round  the 
poles  of  a  magnet.  It  is  not  even  necessary  in 
practice  to  use  lines  at  all ;  dots  will  do 
equally  well,  and  enable  the  formulae  to  be 
written  much  more  concisely.  Begin  with 
an  abstract  case,  and  the  principle  will 
become  clear. 

A  molecule  is  composed,  say,  of  AB2C. 
The  mere  juxtaposition  of  symbols  here 
indicates  chemical  union,  but  the  formula 
does  not  show  how  the  atoms  are  grouped 
— whether  A  is  combined  with  B  or  with 
C,  or  with  both ;  whether  B  is  combined 
with  B  or  with  A  or  C,  and  so  forth.  Suppos- 
ing now  that  it  was  known  definitely  from 
the  mode  of  decomposition  of  the  molecule 
that  C  was  combined  with  A,  and  also  with 
both  atoms  of  B,  then  the  formula  would  more 
nearly  express  the  facts  if  it  were  written 


212  CHEMISTRY 

ACB2.  But  each  of  the  atoms  concerned 
in  building  up  this  molecule  has  its  propel 
valency,  and  can  therefore  be  "  bonded  "  with 
its  associates  in  such  a  way  as  to  indicate 
the  full  structure  or  mode  of  grouping  thus, 

or,   more  briefly,  A:C:B2,  in  which 

formulae  it  will  be  seen  that  A  is  bivalent,  C 
quadrivalent  and  B  univalent.  This  is  a  type 
of  what  is  known  as  a  structural  or  constitu- 
tional formula.  Such  a  formula  is  obviously 
more  expressive  of  the  reality  of  structure 
than  the  empirical  formula  AB2C,  which  tells 
us  nothing  more  than  that  the  molecule 
contains  one  atom  of  A,  two  of  B,  and  one 
of  C.  All  the  compounds  which  have  been  re- 
ferred to  in  former  chapters  can  be  formulated 
on  this  principle.  Thus,  H  -  Cl ;  H  -  O  -  H, 
N-H3,  CnH4,  C  =  0,  O  =  C  =  O,  Fe  =  I2, 
Fe  =  S,  etc.,  'stand  as  structural  formulae 
for  hydrogen  chloride,  water,  ammonia, 
methane,  carbon  monoxide,  carbon  dioxide, 
ferrous  iodide,  ferrous  sulphide.  The  valen- 
cies of  the  atoms  are  evident  in  such  formulae  ; 
not  only  the  simple  valencies  of  elements  in 
which  the  atomic  and  equivalent  weights  are 
identical,  but  also  the  variable  valencies  due 
to  multiple  equivalence,  as  in  the  case  of  the 
above  oxides  of  carbon,  in  which  the?  atom 


CHEMICAL    STRUCTURE         213 

may  be  bivalent  or  quadrivalent,  or  in  sul- 
phuretted hydrogen  and  the  oxides  of  sulphur 
(p.  128)  :— 

H-S-H,    0  =  S  =  0,    OnS°,  in  which 


sulphur    is    bivalent,    quadrivalent,    or    sex- 
avalent. 

Structural  or  constitutional  formulae  must 
obviously  increase  in  complexity  with  increase 
in  the  number  of  atoms  composing  the  mole- 
cules. They  may  be  written  fully  spread  out 
with  all  the  bonds  represented  by  lines,  and 
so  expressed  as  "  graphic  formulae,"  although 
in  practice  the  chemist  soon  becomes  accus- 
tomed to  substitute  dots  for  lines,  and  to 
pack  up  the  symbols  so  as  to  occupy  as  little 
space  as  possible.  The  significance  of  the 
formula  is  in  no  way  impaired  by  such  con- 
densation. It  will  be  realized  that  this  method 
of  formulation  corresponds  with  something 
very  real,  since  it  expresses  the  sum  total  of 
ascertained  fact  —  it  is  the  pictorial  representa- 
tion of  the  mode  of  attachment  of  the  various 
atoms  in  a  molecule  as  ascertained  by  experi- 
ment. The  chemical  structure  of  a  mole- 
cule is  not  a  purely  mathematical  problem  in 
permutations  and  combinations  ;  it  is  some- 
thing more  than  this  —  it  is  a  problem  in 
permutations  and  combinations  controlled 


214  CHEMISTRY 

by  fixed  molecular  architectural  require- 
ments necessitated  by  the  valencies  of  the 
atoms. 

The  conception  of  chemical  structure  is 
obviously  of  the  same  order  as  the  conception 
of  valency  upon  which  it  is  founded — it  is 
descriptive  rather  than  explanatory.  The 
experimental  evidence  upon  which  a  structural 
formula  is  based  is  often  difficult  to  obtain, 
and  still  more  frequently  difficult  to  interpret. 
The  final  aim  of  the  investigator  is  to  be 
able  to  represent  the  atomic  structure  of 
molecules  by  such  formulae.  The  progress 
which  has  been  made  in  this  direction  is 
synonymous  with  the  progress  of  modern 
Chemistry.  By  every  available  method  of 
attack  is  this  problem  approached — by  pulling 
down  and  building  up  by  chemical  methods, 
or  by  comparative  physical  methods  starting 
from  certain  measurable  physical  properties 
correlated  with  simple  compounds  of  known 
structure,  and  extending  the  results  to  mole- 
cules of  greater  and  greater  complexity.  Such 
imperfection  as  exists  in  our  structural 
formulae  is  due  partly  to  imperfect  informa- 
tion concerning  the  actual  mode  of  grouping 
of  the  atoms,  and  partly  to  that  empiricism 
which  at  present  attaches  to  the  conception 
of  valency.  In  a  molecule  composed  of  a  few 


CHEMICAL    STRUCTURE          215 

atoms,  the  possibilities  of  grouping  are  neces- 
sarily limited  ;  when,  as  in  the  case  of  many 
of  the  highly  complex  compounds  of  carbon 
which  build  up  the  tissues  of  living  organisms, 
we  have  molecules  composed  of  hundreds  of 
atoms,  we  have  passed  into  a  new  order  of 
things — a  domain  at  present  beyond  the 
resources  of  structural  formulation. 

So,  also,  with  respect  to  the  graphic  repre- 
sentation of  definite  compounds  of  known 
structure,  there  are  limitations  imposed  by 
our  ignorance  of  the  true  cause  of  valency. 
Thus,  when  a  structural  formula  has  been 
assigned  to  a  particular  compound,  and  all 
the  maximum  valencies  of  the  various  atoms 
have  been  accounted  for,  the  molecule  as  a 
whole  may  still  possess  the  faculty  of  com- 
bining with  other  molecules.  In  other  words, 
molecules,  the  valencies  of  the  atoms  of  which 
are  all  apparently  in  chemical  language 
saturated,  may  still  behave  as  unsaturated. 
Such  molecular  compounds  are  quite  definite, 
but  a  new  order  of  combination  appears  to 
come  into  play.  Many  metallic  salts,  for 
instance,  combine  with  definite  quantities  of 
water  of  crystallization,  or  with  definite 
numbers  of  molecules  of  ammonia  or  other 
compounds.  The  oxide  of  iron  of  "  scale," 
Fe3O4,  may  be  a  molecular  compound  of 


216  CHEMISTRY 

FeO  and  Fe2O3.  Thus  it  has  been  found 
necessary  to  extend  the  main  doctrine  by 
recognizing  residual  affinities,  partial  valencies, 
auxiliary  valencies,  etc.  A  new  field  has,  in 
fact,  been  opened  up  in  this  direction  ;  and 
the  completion  of  the  theory  of  valency  cannot 
be  looked  for  until  this  field  has  been  more 
extensively  cultivated.  The  pioneers  are  at 
work,  and  considerable  advances  have  been 
made  of  late  years. 

But  the  defects  of  the  theory  which  have 
been  indicated  in  no  way  detract  from  its 
utility  as  a  means  of  representing  chemical 
structure  as  far  as  its  applications  can  be 
pushed.  The  bare  recognition  of  the  principle 
that  chemical  character  and  atomic  configura- 
tion are  interdependent  marks  an  epoch  in 
chemical  thought.  Thus,  it  has  long  been 
known  that  certain  elements  are  capable  of 
existing  in  different  forms — the  so-called 
allotropic  modifications.  Oxygen,  for  instance, 
under  the  influence  of  the  silent  electric  dis- 
charge assumes  a  different  and  more  active 
form  known  as  ozone.  In  this  case  there  is 
no  transformation  of  matter ;  ozone  is  still 
at  basis  oxygen.  It  is  known  that  the  change 
in  character  is  due  to  a  difference  in  the  state 
of  aggregation,  the  molecule  of  ozone  being 
triatomic,  O3,  whereas  oxygen  is  diatomic, 


CHEMICAL    STRUCTURE         217 

O2  (p.  160).  This  is  proved  by  the  vapour  den- 
sity of  ozone,  or  by  the  conversion  of  a  known 
quantity  of  ozone  into  oxygen  by  the  action 
of  heat.  Oxygen  being  bivalent  or  quad- 
rivalent, the  structural  formula  of  oxygen 
would  be  O  :  O,  and  of  ozone  (jO'o  or  O  :  O  :  O. 
Carbon,  again,  as  an  element  is  the  same 
"  stuff  "  whether  transparent  and  crystalline, 
as  in  the  diamond,  or  black  and  opaque  as  in 
graphite  and  charcoal.  Phosphorus,  also, 
may  be  an  exceedingly  inflammable  wax-like 
solid  with  a  low  melting-point,  or  a  compara- 
tively inert  reddish  powder  :  The  two  forms 
are  interconvertible. 

Such  cases  are  possibly  explained  by 
differences  in  the  state  of  atomic  aggregation 
—they  may  belong  to  the  same  category  as 
oxygen  and  ozone,  only  the  molecular  weight 
of  solid  carbon  or  red  phosphorus  cannot,  for 
the  reasons  already  set  forth  (p.  172),  at  present 
be  determined ;  and  so  the  actual  numbers 
of  atoms  contained  in  the  different  modifica- 
tions cannot  be  definitely  fixed.  But  inde- 
pendently of  the  question  of  molecular  weight, 
the  conception  of  structure  asserts  itself.  A 
difference  of  molecular  aggregation  in  a  sense 
implies  difference  of  structure ;  but  it  will 
also  be  seen  that  there  might  be  difference  of 
structure  due  to  different  configurations  of 


218  CHEMISTRY 

atoms  within  the  same  molecule.  Sulphur, 
for  example,  exists  in  several  modifications, 
four  solid  and  crystalline,  one  a  yellow  mobile 
liquid  (119°  —  160°),  one  a  dark  viscous  sub- 
stance (160°  -  448-5°),  and  a  vapour  above 
the  latter  temperature.  The  vapour  exists,  as 
we  know  (p.  167),  in  different  states  of  atomic 
aggregation.  The  differences  between  the 
crystalline  forms  are  probably  due  to  differ- 
ences of  molecular  arrangement,  i.e.,  to  physi- 
cal structure ;  the  difference  between  the 
solid  and  the  other  forms  may  be  due  to 
differences  of  atomic  aggregation,  as  with 
oxygen  and  ozone ;  while  the  difference 
between  the  liquid  and  viscous  forms  may  be 
due  either  to  differences  of  atomic  aggregation 
or  to  differences  of  atomic  grouping  within 
the  molecule.  It  is  evident  that  the  decision 
between  molecular  grouping,  atomic  aggrega- 
tion, and  differences  of  intra-molecular  struc- 
ture can  only  be  given  when  the  molecular 
weights  of  the  solid  and  liquid  forms  can  be 
determined. 

The  Chemistry  of  Carbon. — While  the  con- 
ception of  structure  can  at  present  be  borne 
in  mind  in  its  possible  application  to  such 
cases  as  the  above,  the  triumph  of  the  theory 
finds  full  expression  in  the  domain  of  what  is 
called  Organic  Chemistry.  This  term  is  a 


CHEMISTRY    OF    CARBON       219 

survival  from  a  period  when  the  compounds 
dealt  with  under  this  division  were  supposed 
to  be  producible  only  through  living  agency, 
i.e.,  by  organisms  belonging  to  the  animal  or 
vegetable  kingdom.  With  the  progress  of 
science,  this  meaning  of  the  term  "  organic  " 
disappears,  since  large  numbers  of  these 
compounds — identical  with  the  natural  pro- 
ducts— are  now  produced  by  synthetical 
methods  in  our  laboratories,  and  many  of  them 
of  technical  use  are  manufactured  on  a  colossal 
scale.  Organic  Chemistry  is,  in  fact,  the 
chemistry  of  carbon,  since  it  is  this  element 
which,  in  combination  with  a  few  other 
elements,  notably  hydrogen,  oxygen,  and 
nitrogen,  gives  rise  to  the  vast  multitude  of 
compounds  of  every  degree  of  complexity 
which  constitute  the  materials  which,  with 
certain  mineral  or  inorganic  constituents, 
build  up  all  animals  and  plants.  The  ques- 
tion whether  "  vitality  "  may  be  a  function 
of  the  special  chemical  and  physical  properties 
due  to  the  association  of  carbon  with  certain 
other  elements  is  of  profound  interest,  but 
cannot  be  discussed  here.  To  the  chemist, 
organic  matter  is  carbonaceous  matter  ;  mod- 
ern science  knows  of  no  "  vitalism "  apart 
from  carbon  compounds. 

The   division  of  Chemistry  into  inorganic 


220  CHEMISTRY 

and  organic  is  now  only  a  matter  of  con- 
venience arising  from  the  enormous  mass  of 
material  supplied  by  carbon  chemistry.  Over 
150,000  definite  "  organic "  compounds  are 
known,  a  small  fraction  only  of  this  number 
being  the  products  of  "  vitality."  The  re- 
mainder are  all  artificial — synthetical  com- 
pounds unknown  in  nature  till  called  into 
existence  by  the  exercise  of  the  power  of 
chemical  science  over  the  inner  constitution  of 
molecules,  a  power  made  possible  mainly  by 
the  theory  of  structure  based  upon  valency. 
In  recording  this  triumphant  success  of  syn- 
thetical chemistry,  it  must  be  emphasized 
that  there  has  as  yet  been  produced  in  the 
laboratory  no  compound  possessing  in  the 
least  degree  those  characters  which  pertain 
to  living  organic  matter.  The  great  and 
fundamental  problem  of  bridging  the  gap 
between  living  and  dead  matter  still  remains 
unsolved ;  it  may  remain  unsolved  for  all 
time,  or  it  may  not.  For  aught  that  we 
know,  Nature  may  be  solving  this  problem 
daily  under  our  eyes,  but  her  methods  have 
as  yet  remained  unrevealed.  Any  revelation 
in  this  direction  that  awaits  the  science  of 
the  future  must  perforce  be  through  Organic 
Chemistry. 

The  potency  of  structural  formulation  as 


CHEMISTRY    OF    CARBON       221 

a  means  of  giving  expression  to  molecular 
structure  is  manifest  among  carbon  com- 
pounds in  every  direction.  The  applications 
are  so  numerous — the  field  is  so  vast  that 
only  the  barest  hints  can  be  thrown  out  in  this 
volume.  It  will  be  perceived  as  a  general 
principle  that,  with  increase  in  complexity, 
i.e.,  with  increase  in  the  number  of  atoms 
composing  the  molecule,  there  must  result  a 
continually  and  rapidly  augmenting  number 
of  possible  formulae.  Are  these  formulae 
real  ? — does  each  different  configuration  of 
atoms  correspond  with  some  definite  chemical 
compound  ? — in  brief,  how  far  do  such  graphic 
representations  express  the  facts  of  chemical 
science  ? 

The  answer  to  these  questions  carries  with 
it  the  vindication  of  the  claim  of  the  theory  of 
structure  to  run  parallel  with,  if  not  actually 
to  represent,  the  underlying  physical  reality 
of  the  atomic  configuration  of  molecules. 
So  close  is  the  correspondence  between  theory 
and  observation  that,  given  the  number  of 
atoms  composing  a  molecule,  it  is  possible 
to  predict  the  number  of  compounds  that 
ought  to  exist,  with  full  confidence  that  such 
compounds  may  actually  be  obtainable.  In 
hundreds  of  the  simpler  cases,  the  corre- 
spondence of  theory  with  fact  is  complete — 


222  CHEMISTRY 

every  compound  required  by  theory  has  been 
prepared.  There,  thus  emerges  the  great 
principle  of  isomerism,  which  simply  indicates 
that  totally  different  compounds  may  be  built 
up  of  the  same  numbers  of  atoms  of  the 
same  elements.  It  is  not  only  the  total 
number  of>  atoms,  but  the  grouping  of  the 
atoms  within  the  molecule  that  is  the  deter- 
mining cause  of  individuality.  A  phenomenon 
which  at  first  appeared  at  variance  with  all 
common  sense  notions — the  fact  which  stag- 
gered the  early  investigators  in  this  field, 
viz.,  that  two  or  more  molecules  might  have 
precisely  the  same  ultimate  composition,  and 
yet  be  quite  distinct  forms  of  matter,  has 
become  a  commonplace  doctrine  in  modern 
science  in  the  light  of  the  theory  of  chemical 
structure.  Moreover,  the  development  of 
structural  chemistry  has  of  late  years  led  to 
the  recognition  of  the  internal  mobility  of 
atoms  within  molecules — of  structural  con- 
figurations so  delicately  balanced  that  the 
atoms  constituting  the  molecule  may  assume 
one  or  another  of  two  quite  different  configura- 
tions according  to  the  conditions  to  which 
it  is  exposed.  To  this  phenomenon,  the 
general  term  tautomerism  is  applied. 

Stereochemistry. — Yet  another  step  in  the 
theory    of   chemical    structure,    and   we   are 


STEREO-CHEMISTRY  223 

face  to  face  with  one  of  the  most  brilliant  of 
modern  achievements  in  the  direction  of 
bringing  Chemistry  into  the  category  of  the 
deductive  sciences.  The  atoms  composing 
a  molecule  must  obviously  form  a  group  in 
space — the  configuration  is  not  that  of  a 
congeries  of  points  all  lying  in  one  plane,  but 
a  system  occupying  tridimensional  space. 
This  conception  was  first  definitely  applied 
to  the  structural  formulation  of  carbon  com- 
pounds by  Le  Bel  and  van't  Hoff  in  1874. 
The  four  "  bonds  "  of  the  carbon  atom,  for 
example,  may  be  represented  by  lines  radiating 
symmetrically  from  the  carbon  atom  as  a 
centre.  This  is  expressed  geometrically  by 
supposing  that  the  carbon  atom  is  in  the 
centre  of  a  regular  tetrahedron,  the  points  of 
the  angles  of  which  represent  the  terminations 
of  the  bonds  to  which  are  attached  the  other 
atoms  or  groups  of  atoms  which  build  up 
the  molecule.  If  the  carbon  atom,  regarded 
from  this  point  of  view,  is  combined  with  four 
different  atoms  or  groups  of  atoms,  there  then 
arises  as  a  geometrical  necessity  the  existence 
of  two  different  space  groupings  of  the  same 
molecule  which  are  non-superposable,  and 
which  are  related  to  each  other  in  the  same 
way  that  an  object  is  related  to  its  reflected 
image  in  a  mirror — a  right  and  a  left-handed 


224  CHEMISTRY 

isomerism  quite  incapable  of  being  represented 
by  any  formula  which  ignores  the  space  con- 
figuration of  the  atoms.  This  conception  in 
its  modern  developments  may  almost  be  said 
to  complete  the  theory  of  isomerism ;  large 
numbers  of  cases  in  which  the  differences 
between  compounds  of  the  same  ultimate 
composition  cannot  be  expressed  by  plane 
surface  structural  formulae  are  now  known  to 
be  cases  of  stereochemical  isomerism.  This 
newer  development  of  the  atomic  theory — 
known  as  Stereochemistry — is  gradually  per- 
vading and  making  its  influence  felt  through- 
out the  whole  domain  of  our  science.  The 
fundamental  idea  of  space  grouping  is  not 
easy  to  follow  at  first  without  the  aid  of  models, 
but  the  modern  student  is  being  taught  to 
handle  these  formulae  which,  by  virtue  of  their 
rationality,  are  bound  to  dominate  more  and 
more  all  our  notions  concerning  the  structure 
of  molecules. 

The  possibilities  of  isomerism,  regarded 
from  the  stereochemical  point  of  view,  natur- 
ally become  more  complex  with  the  increase 
in  the  number  of  carbon  atoms  which  comply 
with  the  conditions  of  asymmetry  just  defined. 
Here,  again,  is  there  close  parallelism  between 
deduction  from  the  theory  and  observed  facts 
— an  everlasting  testimony  to  the  fertility 


STEREO-CHEMISTRY  225 

of  the  idea.  Thus,  the  acid  of  sour  milk,  lactic 
acid,  contains  an  asymmetric  carbon  atom, 
and  exists  in  two  stereochemical  forms,  as 
required  by  theory ;  tartaric  acid  contained 
in  the  juice  of  grape  and  other  fruits  contains 
two  asymmetric  carbon  atoms,  and  exists  in 
three  stereo-isomeric  forms  as  required  by 
theory.  The  group  of  sugars  typified  by  grape 
sugar,  or  glucose,  comprised  under  the  formula 
C6H12O6,  contain  four  asymmetric  carbon 
atoms,  and  can  exist  in  sixteen  stereo-isomeric 
forms.  Many  of  these  sugars  are  natural  pro- 
ducts ;  and  nearly  the  whole  series  of  sixteen 
required  by  theory  has  been  synthesized  by 
Emil  Fischer  and  his  colleagues — a  veritable 
triumph  of  modern  carbon  chemistry. 

One  of  the  chief  points  of  interest  arising 
from  space  formulation  is  the  correspondence 
between  configuration  and  a  certain  physical 
property,  viz.,  that  of  optical  activity,  by 
virtue  of  which  certain  compounds  possess 
the  power  of  causing  the  rotation  of  polarized 
light  in  either  a  right-handed  (dextro)  or  left- 
handed  (laevo)  direction.  It  is,  in  fact,  by 
this  character  alone  that  the  stereo-isomerism 
is  in  most  cases  revealed,  since  such  isomer- 
ides,  unlike  ordinary  isomerides,  are  alike  in 
all  other  physical  and  chemical  characters.  In 
the  light  of  Stereo-chemistry,  optical  activity 


226  CHEMISTRY 

is  shown  to  be  associated  with  this  intra- 
molecular asymmetry.  Physics,  Chemistry, 
and  Biology  herein  find  another  common 
meeting  ground,  since  it  is  to  Physics  that  we 
look  for  the  explanation  of  the  mechanism  of 
the  connection  between  asymmetry  and  opti- 
cal activity ;  while  the  chemical  processes 
which  go  on  in  the  living  organism  frequently 
result  in  the  apparently  direct  production 
of  optically  active  carbon  compounds — an 
achievement  which  some  chemists  believe  to 
be  an  essential  privilege  of  "  vitalism."  But 
laboratory  syntheses  also  result  in  optically 
active  compounds ;  the  lactic  and  tartaric 
acids,  the  6-carbon-atom  sugars,  and  hosts  of 
other  compounds  have  all  been  synthesized 
in  their  stereo-isomeric  optically  active  forms. 
The  main  difference  is  that  biochemical 
synthesis  is  directive  in  the  sense  of  leading 
to  the  final  production  of  the  optically  active 
compound,  while  laboratory  synthesis  is  at 
present  without  such  directive  power — the 
two  possible  configurations  are  produced 
simultaneously,  and  the  final  product  is,  there- 
fore, optically  inactive  by  compensation.  But 
such  inactive  compounds  can  be  afterwards 
separated  or  resolved  into  their  stereo-iso- 
merides  by  temporary  combination  with  other 
active  compounds  of  vital  origin.  With  the 


CHEMISTRY    OF    CARBON       227 

solution  of  the  fundamental  problem  of  con- 
trolling laboratory  synthesis  so  as  to  suppress 
the  production  of  the  one  or  the  other  of  the 
possible  intra-molecular  configurations,  the 
temporary  aid  of  the  optically  active  vital 
compound  would  be  dispensed  with.  There 
would  then  disappear  another  of  the  barriers 
which  have  been  erected  between  living  and 
dead  organic  matter. 

The  asymmetry  possible  for  a  quadrivalent 
atom,  such  as  carbon,  is  obviously  conceivable 
in  the  case  of  other  atoms.  The  hint  given 
by  carbon  chemistry  has  been  taken  with  all 
the  seriousness  which  attaches  to  what  the 
man  of  science  knows  to  be  a  great  truth. 
Other  quadrivalent  elements,  such  as  tin, 
silicon,  and  sulphur;  quinquevalent  elements 
such  as  nitrogen  and  phosphorus,  and,  quite 
recently,  the  atoms  of  certain  metals  such  as 
cobalt,  chromium,  platinum,  and  rhodium, 
have  been  made  to  form  optically  active 
stereo-isomerides.  The  reader  will  realize 
that  this  new  and  vast  domain  which  is  being 
opened  up  by  many  workers  in  many  lands 
invests  the  atom  of  modern  Chemistry  with  a 
reality  so  vivid  that  the  correspondence 
between  mental  imagery  and  observed  fact 
cannot  be  said  to  be  surpassed  in  any  of  the 
purely  deductive  sciences. 


CHAPTER   X 

THE    PERIODIC    CLASSIFICATION    OF    THE 
ELEMENTS — CONCLUSION 

A  SCIENCE  which,  in  its  purely  materialistic 
aspects,  claims  for  its  subject  matter  the 
study  of  some  eighty  odd  elements,  and  all  the 
compounds  capable  of  being  formed  by  these 
elements,  would  be  but  a  heterogeneous  jum- 
ble of  facts  without  guidance  from  general 
principles.  It  has  only  been  possible  within 
the  compass  of  this  volume  to  give  the  reader 
a  glimpse  here  and  there  into  some  of  these 
principles.  The  treatment  has  perforce  been 
narrowly  materialistic  ;  and  yet  it  must  not 
be  concluded  that  the  chemist  takes  only  into 
consideration  the  matter  of  which  the  universe 
is  composed.  The  energy  associated  with 
this  matter — its  distribution  during  chemical 
change,  the  development  or  absorption  of 
heat  as  concomitants  of  chemical  transforma- 
tions, the  production  of  electricity,  of  light, 
and,  generally,  the  physical  manifestations 

228 


PERIODIC    CLASSIFICATION      229 

resulting  from  this  redistribution  of  energy  are 
as  much  within  the  province  of  modern  Chem- 
istry as  is  the  natural  history  of  the  elements 
and  their  compounds.  An  introductory  frag- 
ment only  has  been  offered  in  the  hope  that  a 
stimulus  may  be  given  to  the  desire  for  fuller 
and  more  specialized  study. 

With  reference  to  matter  as  such,  it  will  be 
readily  seen  that  any  scheme  which  enables 
the  whole  body  of  elements  to  be  grouped  and 
classified  according  to  their  natural  relation- 
ships must  mark  an  advance  towards  the  sys- 
tematization  of  our  knowledge  of  the  highest 
order  of  importance.  That  natural  relation- 
ships exist  among  the  members  of  chemical 
families,  which  possess  certain  characters  in 
common,  and  which  also  show  regular  grada- 
tions of  properties  when  considered  in  series 
arranged  in  the  order  of  their  atomic  weights, 
has  already  been  illustrated  by  reference  to 
the  halogens  (p.  100).  Any  other  family,  non- 
metals  or  metals,  might  have  been  made  to 
enforce  the  same  lesson.  But  such  classifica- 
tion is  restricted  ;  a  wider  and  more  compre- 
hensive scheme  which  embraces  all  the  natural 
groups  or  families  of  elements  was  first  sug- 
gested by  J.  A.  R.  Newlands  in  1864,  and  was 
elaborated  and  put  upon  a  scientific  basis  by 
Mendeleeff  and  Lothar  Meyer  in  1869.  Brief 


230  CHEMISTRY 

|  §  «o  consideration  to  this  scheme  may 

§  2  ,|  g  foe  given  in  this  concluding  chapter. 
If  the  elements  are  arranged  in 

So  co  rl  o  the  order  of  their  atomic  weights, 

§  ~  j| «  a  remarkable  recurrence  will  be 

a,  noticed  after  a  certain  number  of 

!>  3  i  g  elements  have  been  passed  through. 

•^  S  I* «  Thus,  omitting  hydrogen — which 

^  £  stands  in  a  unique  position — a 

g  OT  series   of  eight  members  must  be 

<§"! 


^  arranged  in  order  to  bring  out  the 


fact    that    with    the    ninth    there 
g      .1  ,_,     begins  another  series,  in  which  the 
j$       I  S5     chemical   and   physical    properties 
<j         of  the  first  series  recur  with  that 
modification    due    to    gradational 
transition    illustrated   in   the    case 
e*     of   the  halogens.     The  two   series 
are  given  here  for  comparison. 

Reading  these  lists  in  the  first 
place    in    horizontal    sequence,    it 
will    be    seen    that    there   is    con- 
tinuous  increase  in  atomic  weight 
from   right   to  left.     In    the   next 
2  £  si     place,    it     will     be    noticed     that 
1  If  1  lc    recurrence     °f     characters     takes 
~  £  1  &     place    after    the    eighth    element 
^  'I     (fluorine),     this    recurrence    being 
^      ^     made     evident     by     reading     the 


PERIODIC    CLASSIFICATION     231 

series  vertically — each  vertical  column  as 
here  arranged  consists  of  a  pair  of  elements 
belonging  to  the  same  family.  Now,  if  the 
whole  of  the  chemical  elements  are  arranged 
on  this  principle,  there  results  a  general 
scheme  which  brings  out  the  fact  that  not  only 
are  the  properties  of  the  elements  connected 
with  their  atomic  weights,  but  that  there  is 
periodicity  in  the  relationships.  The  two 
series  given  above  consist  of  eight  members 
each  ;  after  these,  the  periods  become  longer, 
but  the  relationship  between  the  members 
of  the  vertical  columns  is  still  maintained — 
the  grouping  in  these  columns  is  into  natural 
families  with  ascending  atomic  weights  and 
consequent  gradation  of  characters.  The 
law  expressing  this  periodicity  is  known  as  the 
Periodic  Law  ;  and  the  general  scheme  is  the 
Periodic  Classification.  The  Table  (A;  next 
page)  gives  the  arrangement  at  a  glance,  the 
symbols  of  the  elements  being  used  for  the 
sake  of  compactness.  The  atomic  weights 
can  be  supplied  from  the  international  list 
appended  to  this  chapter. 

The  first  point  to  which  attention  is  directed 
is  naturally  the  periodic  arrangement,  which 
is  the  basis  of  the  whole  scheme.  The  Roman 
numerals  in  the  first  horizontal  column  are 
group  numbers  labelling  the  groups  below 


232 


CHEMISTRY 


o 

u 


2 


I 


PQ 


O 


c8 
CO 


I 

I 


cc 


Q 

Q 


CO 


0 


c8 

O 


C/3 


N 


PQ 


S3 


W 


J-l 
CO 


PQ 


tf 


bo 


I 


CM 

O 


w 


PERIODIC    CLASSIFICATION      233 

them  in  the  vertical  columns.  The  first  two 
(short)  periods,  consisting  of  eight  members 
each,  are  given  on  p.  232.  The  third  (long) 
period  begins  with  argon,  and  ends  with 
bromine ;  the  fourth  (long)  period  begins 
with  krypton,  and  ends  with  iodine.  It  will 
be  noticed  that  in  these  cases  the  elements  of 
the  long  periods  are  arranged  in  two  horizontal 
series,  so  as  to  bring  the  members  of  the  same 
family  into  their  respective  groups  (vertical 
columns).  After  the  fourth  group,  the  sys- 
tematic uniformity  is  no  longer  maintained, 
because,  following  lanthanum,  there  has  to  be 
interpolated  a  whole  cohort  of  elements  which 
are  most  closely  related  among  themselves. 
These  are  the  so-called  "  rare  earth  "  metals  ; 
and  for  the  sake  of  compactness  they  are  sim- 
ply inserted  en  bloc  in  the  order  of  their  atomic 
weights.  The  list  of  these  metals  is  probably 
still  incomplete ;  new  members  may  be 
isolated  as  the  result  of  further  research. 
The  known  metals  of  this  group  exist  as 
compounds  in  a  complex  mixture  of  minerals 
known  as  monazite  sand,  which  is  found  in 
various  parts  of  the  world,  and  which  is 
worked  up  on  the  large  scale  for  the  manufac- 
ture of  the  earthy  mantles  used  as  incandes- 
cent gas  burners.  These  mantles  are  com- 
posed mainly  of  the  oxides  of  thorium  and 


234  CHEMISTRY 

cerium,  supported  by  a  suitable  framework. 
After  these  elements,  the  horizontal  series 
become  fragmentary — they  are  suggestive  of 
incomplete  periods. 

Considering  the  scheme,  in  the  next  place, 
with  reference  to  the  natural  families  or 
groups  in  the  vertical  columns,  it  will  be 
seen  that  the  relationships  are  more  faithfully 
expressed  by  a  further  division  into  two  sub- 
groups. This  simply  indicates  that,  while 
the  members  of  the  group  as  a  whole  are 
closely  related  chemically,  there  is  a  still 
closer  inner  relationship  between  the  members 
of  the  sub-group.  These  points  will  be  made 
clear  by  a  few  examples.  Thus,  in  Group  I. 
will  be  found  the  very  natural  family  of  metals, 
beginning  with  lithium  and  ending  with 
caesium,  known  as  the  alkali  metals.  Their 
oxides  form  strongly  basic,  alkaline  solutions 
(p.  108) ;  and  they  all  form  similar  types  of 
compounds.  The  sub-group — copper,  silver 
and  gold — is  related  to  the  alkali  metals  by 
virtue  of  certain  types  of  compounds  which 
the  metals  named  are  capable  of  forming ; 
but  the  relationship,  although  confessedly 
not  very  close,  is  more  intimate  between  the 
three  metals  themselves — they  resemble  each 
other  more  closely  in  their  general  characters 
than  they  do  the  metals  of  the  alkalis.  In 


PERIODIC    CLASSIFICATION      235 

Group  II.,  again,  will  be  found  the  metals  of 
the  "alkaline  earths  "  (calcium,  barium,  etc.), 
the  oxides  of  which  are  alkaline  earthy  sub- 
stances typified  by  lime  (p.  102).  The  metals 
of  the  sub-group  (glucinum,  magnesium,  etc.) 
are  naturally  related  to  these,  but  still  more 
closely  related  to  each  other.  The  halogens 
(p.  100),  as  will  be  seen,  fall  into  Group  VII., 
and  are  (somewhat  remotely)  related  to 
manganese. 

The  general  nature  of  the  scheme  should  be 
made  evident  by  these  examples.  It  will  be 
seen  that,  on  the  whole,  chemical  dissimilarity 
characterizes  the  members  of  the  horizontal 
series  until  the  period  recurs.  It  would 
appear  as  though  something  added  to  the  mass 
of  the  atom  caused  it  to  differ  qualitatively 
in  its  chemical  characters  from  its  left-hand 
neighbour  until  a  certain  number  of  addi- 
tions had  been  made,  when  the  difference 
becomes  quantitative  instead  of  purely  qualita- 
tive. And  yet  this  principle  is  not  generally 
complied  with.  The  triplets  of  Group  VIII., 
for  example,  are  separated  out  because, 
among  other  reasons,  they  show  close  re- 
semblance of  character  among  themselves. 
Still  more  closely  related  among  them- 
selves are  the  metals  of  the  "  rare  earths," 
the  separation  of  which  has  necessitated 


236  CHEMISTRY 

the  most  laborious  work  carried  on  for 
very  many  years.  In  these  cases,  there  is  no 
abrupt  change  of  character  in  passing  from 
element  to  element  along  the  horizontal 
series.  Other  discrepancies  occur — notably 
with  the  elements  argon  (39.88)  and  potassium 
(39.1),  which  are  out  of  place  if  the  strict  order 
of  atomic  weights  is  followed.  Iodine  (126.92) 
and  tellurium  (127.5),  have  likewise  to  be 
taken  out  of  order  to  bring  them  into  the 
groups  to  which  they  naturally  belong.  It 
may  be  that  such  discrepancies  indicate  that 
the  atomic  weights  need  further  revision. 

Passing  over  these  and  certain  minor 
discrepancies  as  problems  yet  awaiting  solu- 
tion, the  significance  of  the  scheme  as  a 
comprehensive  whole  must  be  fully  realized  in 
order  that  its  systematizing  influence  upon 
our  science  may  be  adequately  appreciated. 
That  it  corresponds  with  some  underlying 
reality  is  made  manifest  by  the  coincidence  of 
periodicity  in  several  distinct  sets  of  charac- 
ters. The  horizontal  series,  for  instance,  clas- 
sified originally  simply  in  the  order  of  the 
atomic  weights,  will  be  found  to  correspond 
also  with  a  classification  according  to  valency — 
due  allowance  being  made  for  the  variability  of 
this  property  (p.  206).  Measuring  the  valency 
of  the  atom  by  the  maximum  number  of 


PERIODIC    CLASSIFICATION     237 

hydrogen  atoms  with  which  it  combines  if  it 
forms  hydrogen  compounds,  or  with  halogens 
if  it  forms  no  hydrogen  compounds,  the 
valencies  of  the  atoms  of  the  first  series 
may  be  indicated  by  attaching  small  Roman 
numerals  to  the  symbols  : — 

He0;  Li1;   Glu  ;   Bm  ;   Civ  ;   Nui ;   Ou  ;    F1. 

The  periodicity  here  shown  is  observed 
also  in  a  precisely  similar  way  in  the  next 
series.  If  the  valency  is  measured  by  the 
number  of  oxygen  atoms  in  the  acid  and  basic 
oxides  in  any  series  which  comprises  such  a 
set  of  oxygen  compounds,  a  regular  increase 
is  observed  : — 

2nd  Series  :— Ne°  ;   Na^O11 ;    MguOu  ;  Al^Oj} ; 
SiivO£ ;  PIOj*  ;  SviO!j ;   Cl™  O?. 

The  scheme  thus  emphasizes  the  association 
of  chemical  character  with  valency — distinct- 
ness marking  the  transition  from  member 
to  member  along  the  horizontal  series  and 
similarity  among  the  natural  groups  in  the 
vertical  columns.  In  general  terms,  the  Roman 
numerals  heading  the  eight  columns  may  be 
regarded  as  indexes  of  valency — not,  of  course, 
in  too  rigid  a  sense,  but  as  indicating  maximum 
valency.  The  zero  sign  over  the  first  column 
is  to  be  taken  literally  as  meaning  no  valency. 
The  elements  from  helium  downwards  form 


238  CHEMISTRY 

no  compounds  with  other  elements  ;  they  are 
all  chemically  inert  monatomic  gases  (p.  170) 
contained  in  minute  quantities  in  atmospheric 
air  (p.  46).  Their  separation  has  necessarily 
been  effected  by  physical  methods,  since, 
chemically  speaking,  they  are  dead  matter. 
Helium,  it  may  be  mentioned  incidentally, 
is  present  in  the  atmosphere  of  the  sun,  and 
has  been  found  in  small  quantities  in  some 
terrestrial  minerals  and  also  dissolved  in 
certain  mineral  waters.  The  history  of  this 
element  is  intimately  connected  with  the 
subject  of  radioactivity. 

The  key-note — periodicity — having  been 
struck,  the  characters  of  the  elements  generally 
will  be  found  to  respond.  Notice  how  each 
period  begins  with  an  inert  element,  and  then 
passes  through  a  series,  one  extreme  of  which 
is  an  intensely  electro-positive  base-forming 
metal,  and  the  other  extreme  an  intensely 
electro-negative  acid-forming  non-metal  (halo- 
gen), the  extremes  being  connected  by  inter- 
mediate elements  of  decreasingly  electro- 
positive and  increasingly  electro-negative 
characters. 

Another  physical  property  of  the  ele- 
ments, viz.,  their  specific  gravity,  can  also 
be  shown  to  be  conformable  with  the  periodic 
classification.  This  conformability  is  most 


PERIODIC   CLASSIFICATION     239 

strikingly  revealed  by  taking,  instead  of  the 
specific  gravity,  the  number  obtained  by 
dividing  the  atomic  weight  by  the  specific 
gravity — the  so-called  "  atomic  volume," 
which  may  be  regarded  as  the  volume  occu- 
pied by  the  atom  of  the  solid  or  liquid  element. 
The  reader  must  be  careful  to  distinguish 
between  this  "  atomic  volume  "  of  solid  and 
liquid  elements  and  the  volumes  occupied  by 
the  atomic  weights  of  the  elements  in  the 
gaseous  state  (p.  166),  in  which  state  only  does 
the  hypothesis  of  Avogadro  (p.  153)  apply.  If 
the  numbers  representing  the  atomic  weights 
(arranged  in  ascending  order)  are  taken  as 
abscissae  and  the  numbers  representing  the 
atomic  volumes  as  ordinates,  the  points  of 
intersection,  when  joined  up  in  the  usual  way 
familiar  in  co-ordinate  geometry,  give  a  curve 
which  reveals  the  periodicity  at  a  glance. 
The  curve  thus  constructed  has  a  wave-like 
form ;  and  the  chemically  related  elements 
of  the  natural  families  occupy  corresponding 
positions  on  the  curve.  Thus,  the  strongly 
electro-positive  alkali  metals  of  Group  I.  are 
at  the  wave  summits,  the  electro-negative 
halogens  of  Group  VII.  at  corresponding 
positions  on  the  ascending  slopes,  the  alkaline 
earthy  metals  of  Group  II.  on  the  descending 
slopes,  and  the  high  melting-point  metals  of 


240  CHEMISTRY 

lower  chemical  activity  belonging  chiefly  to 
Group  VIII.  in  the  hollows  between  the 
waves. 

Further  details  arising  from  the  periodic 
classification,  the  critical  discussion  of  its 
imperfections,  and  the  various  suggested 
amendments  must  be  sought  for  in  the  stand- 
ard works.  The  value  of  the  scheme — apart 
from  philosophical  considerations — is  due  not 
only  to  its  systematizing  influence,  but  quite 
as  much  to  its  suggestiveness.  It  not  only 
consolidates  existing  knowledge,  but  it  has 
indicated  and  still  points  to  gaps  in  the 
series  which  may  at  some  future  time  be 
filled.  In  other  words,  where  the  small 
increment  in  the  numbers  representing  suc- 
cessive atomic  weights  suddenly  becomes  large, 
a  hint  is  given  that  one  or  more  elements 
yet  remain  to  be  discovered,  or  that  some  of 
the  elements  do  not  occur  on  this  earth. 
Moreover,  since  all  the  properties  of  the 
elements  and  their  compounds  are  gradational 
in  the  ascending  series  of  natural  groups  or 
families,  and  since  these  families  all  fall  into 
the  general  scheme,  it  is  possible  to  predict 
within  narrow  limits  the  properties  of  missing 
elements.  Two  examples  of  such  gaps  are 
inserted  in  the  form  of  queries  in  the  tabular 
scheme  in  Group  VII.,  where  there  is  an 


PERIODIC    CLASSIFICATION      241 

indication  of  an  element  having  an  atomic 
weight  intermediate  between  molybdenum 
(96)  and  ruthenium  (101-7)  and  belonging  to 
the  sub-group  containing  manganese  ;  and  of 
another  element  belonging  to  the  same 
sub-group  between  tungsten  (184)  and  osmium 
(190-9).  Hydrogen,  which,  as  already  stated, 
stands  at  present  alone,  might  be  regarded 
as  the  only  known  representative  of  a 
series,  the  members  of  which,  both  of  higher 
and  (possibly)  of  lower  atomic  weight,  are 
missing. 

A  scheme  which  harmonizes  so  many 
distinct  sets  of  properties  might  legitimately 
be  used  deductively.  The  prediction  of  the 
properties  of  missing  elements  and  their  com- 
pounds illustrates  such  use  ;  and  the  triumph 
of  the  scheme  dates  from  the  period  when 
certain  of  the  gaps  were  filled  in  by  newly- 
discovered  elements,  the  properties  of  which 
— as  foretold  by  Mendeleeff — were  found  to 
correspond  closely  with  the  prediction.  Thus, 
gallium  (Lecoq  de  Boisbaudran,  1875),  scand- 
ium (Nilson,  1879),  and  germanium  (Winkler, 
1886)  all  found  appropriate  niches  awaiting 
them  on  their  discovery.  So  also,  used 
deductively,  the  classification  has  led  both 
to  the  revision  of  atomic  weights  and  to  the 
resortment  of  elements,  i.e.,  to  the  transference 
Q 


242  CHEMISTRY 

of  elements  from  positions  where  there  was  no 
gap  to  be  filled  to  positions  where  they  were 
wanted.  This  is  tantamount  to  determining 
the  group  to  which  a  doubtful  element  belongs, 
and  so  to  the  determination  of  its  natural  rela- 
tionships and  its  valency.  The  scheme  thus 
becomes  available  as  another  method  of 
control  for  deciding  in  doubtful  cases  the 
relationship  between  the  equivalent  and  the 
atomic  weight  (p.  173). 

In  its  philosophical  aspects,  the  periodic 
classification  is  naturally  suggestive  of  evolu- 
tion. Clearly,  the  elements  have  not  been 
launched  haphazard  into  existence  as  inde- 
pendent entities :  the  contemplation  of  their 
various  relationships  and  inter-relationships 
causes  to  arise  spontaneously  the  question 
whether  the  known  forms  of  elemental  matter 
may  not  be  genetically  related — whether  the 
regularities  arising  from  the  successive  addi- 
tions to  the  mass  of  the  atom  may  not  be 
interpreted  in  terms  of  the  development  of  the 
elements  from  some  primordial  "  stuff."  It 
may  be  so — it  would  only  be  in  harmony  with 
the  whole  scheme  of  Nature  that  such  should 
be  the  case.  The  consideration  of  the  various 
attempts  which  have  been  made  to  prove 
elemental  genesis  would,  however,  take  us 
too  far  into  the  region  of  speculation.  The 


PERIODIC    CLASSIFICATION      243 

"  Periodic  Law "  as  it  stands  is,  in  strict 
philosophical  terms,  but  an  empirical  summary 
—its  interpretation  has  yet  to  be  found.  Is 
it  too  great  a  stretch  of  prophecy  to  suggest 
that  the  ultimate  coalescence  of  Physics  and 
Chemistry  will  be  brought  about  through  the 
interpretation  of  the  principle  of  periodicity  ? 
The  actual  evidence  in  support  of  genetic 
relationship  is  at  present  scanty  ;  but  it  is 
slowly  and  surely  accumulating  in  the  field  of 
radioactivity.  Here,  again,  the  scheme,  as 
far  as  it  goes,  harmonizes  with  the  latest 
discoveries.  The  three  elements  of  highest 
atomic  weight — radium,  thorium  and  uranium, 
with  possibly  "  radium  emanation  "  or  niton — 
fall  naturally  into  in  the  last  series.  That 
series  may  be  incomplete — there  are  gaps 
which  may  or  may  not  be  filled  up  by  future 
discovery  ;  but  it  is  in  this  series  that  the 
atoms  appear  to  have  reached  the  limit  of 
internal  stability.  It  is  these  elements  which 
are  undergoing  that  process  of  spontaneous 
decay  or  disintegration  with  the  liberation  of 
the  enormous  store  of  energy  which  manifests 
itself  in  the  phenomena  of  radioactivity.  The 
final  material  product  of  this  atomic  disinteg- 
ration which  has  thus  far  been  definitely 
identified  is  the  inert  gas  helium.  It  is  in  this 
sense,  as  already  stated  (p.  90),  that  transmu- 


244  CHEMISTRY 

tation,  as  distinguished  from  transformation, 
receives  recognition  in  modern  Chemistry. 


CONCLUSION. — The  reader  who  has  followed 
the  development  of  the  subject  up  to  this 
stage  will  now  realize  that  he  has  been 
brought  to  the  threshold  of  a  great  edifice 
crowded  with  departmental  chambers.  The 
various  compartments  are  not  watertight 
divisions — there  is  free  intercommunication 
by  means  of  cross  passages  more  or  less  broad 
and  numerous,  according  to  the  particular 
label  on  the  door  of  the  chamber.  In  this 
small  volume  it  has  not  been  possible  to  do 
more  than  to  point  to  some  of  these  labels  in 
the  hope  that  the  reader  may  be  tempted  to 
explore  in  greater  detail  the  contents  of  the 
various  chambers.  The  inscriptions  on  a  few 
of  these  unopened  doors  are  worthy  of  being 
noted. 

The  redistribution  of  energy  which  accom- 
panies chemical  change  is  dealt  with  in  works 
on  Thermochemistry.  When  heat  is  developed 
as  the  result  of  chemical  combination,  such  as 
when  sulphur  and  iron  combine  (p.  39),  or 
when  carbon  burns  in  oxygen  to  carbon  dioxide 
(p.  125),  and,  generally,  in  all  kinds  of  burning 
or  combustion,  the  products  of  such  combina- 


CONCLUSION  245 

tion  are  said  to  be  exothermic.  Combustion  is 
in  fact  energetic  oxidation.  The  develop- 
ment of  heat  in  such  cases  means  that  the 
products  of  combustion  contain  less  energy 
than  the  materials  which  combine — that  the 
system  has  run  down  in  energy  to  the  extent 
represented  by  the  heat  evolved.  On  the 
other  hand  there  are  compounds  that  can  only 
be  formed  from  their  elements  when  energy  is 
supplied  from  without,  because  the  products 
contain  more  energy  than  their  components. 
These  are  known  as  endothermic  compounds. 
And  thus  we  enlarge  our  conception  of  chemical 
change  by  associating  the  material  transforma- 
tion with  the  accompanying  redistribution  of 
energy.  Moreover,  the  heat  evolved  or  ab- 
sorbed is  as  fixed  and  definite  in  quantity  for 
each  particular  chemical  change  as  is  the 
weight  of  the  matter  concerned.  The  change 
2H2-|- O2=  2H2O  to  the  chemist  means  the 
development  of  136,800  calories  (p.  173)  quite 
as  explicitly  as  it  does  that  four  parts  by  weight 
of  hydrogen  and  32  parts  of  oxygen  give  36 
parts  of  water.  Thus  we  come  once  again 
into  the  presence  of  the  chemical  atom,  now 
as  a  definite  store  of  available  energy  as  well 
as  a  material  particle  possessing  definite 
weight. 

As  the  result  of  the  study  of  chemical  change 


246  CHEMISTRY 

from  the  above  point  of  view  a  quantitative 
measure  of  chemical  activity  has  been  pro- 
vided. The  "  heat  of  formation "  of  com- 
pounds becomes  a  measure  of  the  activities  of 
the  atoms  of  the  combining  elements.  It  must 
be  noted  however  that  the  heat  evolved  repre- 
sents but  a  small  fraction  of  the  total  internal 
energy  of  the  atoms.  In  no  ordinary  chemical 
change  can  it  be  said  that  there  is  tapped 
more  than  a  definitely  limited  quantity  of  this 
vast  store — the  greater  part  of  the  energy 
locked  up  in  the  chemical  atom  is  still  unavail- 
able. The  practical  solution  of  the  problem 
of  liberating  this  store  of  energy — if  it  is  ever 
solved — would  mark  the  dawn  of  a  new  era 
for  the  human  race. 

Then  again  with  respect  to  the  conditions 
which  determine  chemical  change  there  is  a 
great  field  which  we  have  left  untrodden. 
Some  of  the  apparently  simplest  cases  of 
direct  combination  reveal  their  inner  com- 
plexity through  the  fact  that  water  vapour 
— if  only  a  minute  trace — is  essential  for  the 
reaction.  Thus  2H2  +  O2  =  2H2O  ;  H2  +  C12 
=  2HC1;  and  even  NH3+ HC1^±NH4C1  (p. 
169)  represent  chemical  changes  which  do  not 
take  place  when  the  gases  are  absolutely  dry. 
Other  chemical  changes  which  at  first  sight 
would  appear  to  be  quite  simple,  such,  e.g., 


CONCLUSION  247 

as  SO2  +  O  =  SO3  (p.  207),  take  place  only  in 
the  presence  of  heated  finely  divided  platinum 
or  other  metals  which  remain  unchanged  at 
the  end  of  the  reaction.  Metals  and  other 
substances  which  exert  this  mysterious  in- 
fluence are  said  to  act  by  contact  or  cata- 
lytically.  It  may  be  that  the  presence  of  a 
catalyst  of  some  kind  is  a  necessary  condition 
of  all  chemical  change. 

Among  other  unconsidered  conditions  of 
chemical  change  is  the  influence  of  the  active 
masses  of  the  reacting  materials  upon  the 
velocity  of  the  reaction  (p.  73),  upon  the 
direction  in  which  the  change  takes  place  in  a 
reversible  reaction  such  as  3Fe  4-  4H2O  ^  Fe3 
O4+  4H2  (pp.  187  and  198),  and  in  determining 
the  actual  quantities  of  the  products  present 
when  the  system  reaches  equilibrium  under 
various  conditions,  i.e.,  when  the  reversible 
reaction  becomes  balanced  owing  to  the  velo- 
cities of  the  change  in  each  direction  being 
equal.  The  contents  of  the  chambers  la'belled 
Chemical  Statics  and  Dynamics  are  well 
worthy  of  detailed  exploration,  for  here  the 
reader  will  find  that  modern  Chemistry  has 
been  brought  within  the  reach  of  mathematical 
treatment. 


248  CHEMISTRY 

1912. — International  Atomic  Weights. 


Aluminium  .... 
Antimony     .... 

O  =  16. 
Al     27-1 
Sb   120-2 
A      39-88 
As     74-96 
Ba  137-37 
Bi   208-0 
B      11-0 
Br     79-92 
Cd  112-40 
Cs    132-81 
Ca     40-07 
C       12-00 
Ce   140-25 
Cl      35-46 
Cr     62-0 
Co     58-97 
Cb     93-5 
Cu     63-57 
Dy  162-5 
Er  167-7 
Eu  152-0 
F       19-0 
Gd  157-3 
Ga    69-9 
Ge     72-5 
Gl       9-1 
Au  197-2 
He      3-99 
H        1-008 
In    114-8 
I      126-92 
Ir    193-1 
Fe     55-84 
Kr    82-92 
La  139-0 
Pb  207-10 
Li       6-94 
Lu  174-0 
Mg    24-32 
Mn    54-93 
Hg  200-6 
Mo    96-0 

Neodymiuin  .  . 
Neon  

0 

Nd 
Ne 
Ni 
Nt 
ition) 
N 
Os 
0 
Pd 
P 
Pt 
K 
Pr 
Ra 
Rh 
Rb 
Ru 
Sa 
Sc 
Se 
Si 
Ag 
Na 
Sr 
S 
Ta 
Te 
Tb 
Tl 
Th 
Tm 
Sn 
Ti 
W 
U 
V 
Xe 
Yb 
i) 
Yt 
Zn 
Zr 

=  16. 
144-3 

20-2 
58-68 
222-4 

14-01 
190-9 
16-00 
106-7 
31-04 
195-2 
39-10 
140-6 
225-95 
102-9 
85-45 
101-7 
150-4 
44-1 
79-2 
28-3 
107-88 
23-00 
87-63 
32-07 
181-5 
127-5 
159-2 
204-0 
232-4 
168-5 
119-0 
48-1 
184-0 
238-5 
51-0 
130-2 
172-0 

89-0 
65-37 
90-6 

Nickel  

Niton  

(radium  eman, 
Nitrogen     .... 
Osmium  

Bismuth      .    . 

Boron      

Bromine    

Oxvsen  . 

Cadmium  

Palladium  .... 
Phosphorus     .  . 
Platinum     .... 
Potassium  .... 
Praseodymium 
Radium  
Rhodium     .... 
Rubidium   .... 
Ruthenium     .  . 
Samarium  .... 
Scandium    .... 
Selenium 

Cspsium 

Carbon  

Cerium  •  .  . 

Chlorine    

Chromium    .... 
Cobalt 

Columbium  .... 
Copper  

Dysprosium  
Erbium 

Europium     .... 
Fluorine    .  .    . 

Silicon 

Silver  

Gadolinium  .... 
Gallium     

Sodium  

Strontium  .... 
Sulphur 

Germanium  .... 
Glucinuin 

Tantalum  .... 
Tellurium  .... 
Terbium  

Gold         .... 

Helium  

Hydrogen     .... 
Indium        . 

Thallium  
Thorium 

Iodine        .... 

Thulium  .  . 

Iridium      

Tin  

Titanium  
Tungsten  .... 
Uranium  .... 
Vanadium  .... 
Xenon  

Krypton 

Lanthanum  .... 
Lead  

Lithium     

Lutecium 

Ytterbium  
(Neoytterbiun 
Yttrium 

Magnesium   .... 
Manganese    
Mercury    
Molybdenum    .  . 

Zinc  .... 

Zirconium  .... 

BIBLIOGRAPHY 

HlSTOBIOAL 

History  of  Chemistry  /    by  Sir  T.  E.  Thorpe.     Watts  & 

Co.  A  concise  resume. 
Essays  in  Historical  Chemistry  s  by  the  same  author. 

Macmillan  &   Co.     A  series  of  biographies  of  the 

founders  of  the  science. 
A  History  of  Chemistry  from  earliest  times  to  the  present 

day,    being   also    an    Introduction    to   the    Study   of 

the  Sciences    by   Ernst  v.  Meyer,  translated  by  G. 

Mc'Gowan.     Macmillan  &  Co.     A  larger  and  fairly 

exhaustive  work. 

GENERAL,  PHYSICAL,  AND  INORGANIC  CHEMISTRY 

Introduction  to  Chemistry  /  by  W.  Ostwald,  translated 
by  Hall  &  Williams.  Wiley  &  Sons.  A  very  lucid 
elementary  work. 

Outlines  of  General  Chemistry ;  by  the  same  author, 
translated  by  W.  W.  Taylor.  Macmillan  &  Co.  A 
general  statement  of  the  existing  state  of  knowledge 
of  the  science  suitable  for  advanced  readers. 

Theoretical  Chemistry  from  the  Standpoint  of  Avogadro's 
Rule  and  Thermodynamics :  by  W.  Nernst,  trans- 
lated by  H.  T.  Tizard.  Macmillan  &  Co.  An 
advanced  work  dealing  with  the  subject  more 
mathematically. 

249 


250  BIBLIOGRAPHY 

First  Principles  of  Chemical  Theory  :  by  C.  H.  Mathewson. 
Wiley  &  Sons.  A  concise  introduction  to  theoretical 
chemistry. 

Inorganic  Chemistry :  by  E.  J.  Lewis.  Cambridge 
University  Press. 

Text- Book  of  Inorganic  Chemistry  :  by  Holleman,  trans- 
lated by  Cooper.  Wiley  &  Sons. 

Good  introductory  works   to   this   branch  of  the 
science. 

Text-  BooTcs  of  Physical  Chemistry :  by  various  authors, 
edited  by  Sir  Win.  Ramsay.  Longmans,  Green  & 
Co.  A  useful  series  of  monographs,  each  volume 
dealing  with  some  special  branch  of  the  subject. 

STEREOCHEMISTRY 

The  Arrangement  of  Atoms  in  Space :  by  J.  H.  van't 
Hoff,  translated  by  Arnold  Eiloart.  Longmans, 
Green  &  Co.  A  work  of  great  historical  interest 
as  representing  the  views  of  one  of  the  founders 
of  this  subject  and  of  his  disciples. 

Stereochemistry:  by  A.  W.  Stewart.  Longmans,  Green 
&  Co.  One  of  the  Text-Books  of  Physical  Chemistry. 

ORGANIC  CHEMISTRY 

The  Rise  and  Progress  of  Organic  Chemistry  t  by  C. 
Schorlemmer.  Macmillan  &  Co.  A  concise  historical 
introduction. 

Organic  Chemistry  :  by  W.  H.  Perkin  and  F.  S.  Kipping. 

R.  &  W.  Chambers. 
Text- Book  of  Organic  Chemistry  :  by  Holleman,  edited  by 

A.  J.  Walker  and  O.  E.  Mott.     Wiley  &  Sons. 
Good  systematic  introductory  works. 


BIBLIOGRAPHY  251 

Modern  Organic  Chemistry  :  by  C.  A.  Keane.  The  Walter 
Scott  Publishing  Co.  A  good  elementary  exposition 
of  the  later  developments  of  this  branch. 

Organic  Chemistry  for  Advanced  Students :  by  J.  B. 
Cohen.  Edward  Arnold.  A  more  elaborate  historical 
treatment  of  the  subject. 

Modern  Research  in  Organic  Chemistry  :  by  P.  G.  Pope. 
Methuen  &  Co.  A  concise  account  of  the  newer 
lines  of  research. 

APPLIED  CHEMISTRY 

Chemistry  in  Daily  Life  :  by  Lassar-Cohn,  translated  by 

Pattison  Muir.     Grevel  &  Co. 
The   Romance  of  Modern   Chemistry  /    by  J.   C.  Philip. 

Seeley  &  Co. 
The   Chemistry  of  Commerce :   by  R.  Kennedy  Duncan. 

Harper  &  Bros. 
Chemical    Research    and    National     Welfare :    by   Emil 

Fischer.     Society  for  Promoting  Christian  Knowledge. 
Four  popular  works  giving  an  account  of  the  part 

played  by  Cliemistry  in  the  arts  and  manufactures  and 

in  human  \v  elf  are  generally, 

NOTE  : — No  attempt  has  been  made  to  include  in  this  list 
more  than  a  few  selected  types  of  the  multitudinous 
elementary  and  advanced  text- books  written  for 
students.  The  great  standard  works  of  reference 
familiar  to  all  workers  in  Chemistry  would  naturally 
be  beyond  the  scope  of  the  ordinary  reader  who  is 
only  desirous  of  making  himself  acquainted  with  the 
existing  state  of  the  science. 


INDEX 


ACETYLENE,  125 

Acid  oxides,  108 

Activity,  Chemical,  83,  103 

Agencies,  Chemical,  92 

Aggregation,  Physical  state,  45 

Agriculture  and  chemistry,  21 

Air,  Composition  of,  46 

Alchemists,  90 

Alkali  metals,  234     ' 

Alkaline  earths,  235 

Allotropic  modification,  216 

Alloys,  106 

Aluminium,  102 

Ammonia,  152 

Analysis,  93 

Arithmetic,  Chemical,  195 

Art  of  chemistry,  39 

Association,  166 

Asymmetric  atoms,  223 

Atmospheric  moisture,  35 

Atomic  heat,  174 

Atoms  in  molecules,  163 

Atomic  theory,  129 

volumes,  166,  239 

weights,  143,  164,  183 

Auxiliary  valency,  216 
Avogadro's  hypothesis,  157 

Balance,  Chemical,  191 
Barlow  and  Pope,  209 
Basic  oxides,  108 
Bell-metal,  106 
Biochemical  synthesis,  226 
Boiling-point,  59,  178 
Bonds,  211 
Boyle's  law,  148 
Brass,  106 
Bromine,  100 

Calcium,  102 
Calorie,  173 
Cannizzaro,  158 
Carbon  dioxide  in  air,  36 
Carbon  compounds,  220 
. ,  Oxides  of,  125 


253 


Catalysts,  247 

Centigrade  thermometer,  59 

Cerium,  234 

Chalk,  changed  to  lime,  39 

Charcoal,  217 

Charles'  law,  148 

Chemical  change,  21 

combination,  41 

equivalents,  120 

Chili  saltpetre,  101 
Chlorine,  100 
Classification,  100 
Coal-tar,  43 
Coinage,  Gold,  107 

,  Silver,  106 

Combination,  Direct,  29 

• ,  Quantitative,  66 

Combustion,  244 

Compounds,   Equivalence  of,  121 
Copper  oxides,  84,  125 
Constancy  of  composition,  117 
Conservation  of  energy,  89 

• of  mass,  76 

Contact  action,  247 
Crookes,  133 
Cryolite,  100 
Crystallography,  210 

Dalton,  129 

Decomposition,  Quantitative,  66 

Definiteness  of  chemical  change,  71 

Densities  of  gases,  154 

Depression  of  freezing-point,  178 

Dew,  54 

Dewar,  148 

Diamond,  217 

Diffusion  of  gases,  56 

Discrete  structure  of  matter,  58 

Disintegration,  Atomic,  243 

Dissociation,  166 

Double  decomposition,  202 

Dulong  and  Petit's  Law,  173 

Dyea  from  tar,  43 

Earth's  crust,  Composition  of,  114 


254 


INDEX 


Effervescence,  38 
Electro-chemical  equivalence,  121, 

127 

Electro-negative  elements,  111 
Electro-positive  elements,  111 
Electrolysis,  109 
Electrons,  134 
Elemental  genesis,  242 
Elements,  Chemical,  95 
Endothermic  compounds,  245 
Exothermic  compounds,  245 
Explosives  from  tar,  43 
Expansion  by  heat,  22 
Experimental  method,  31 
Equations,  165,  198 
Equivalence,  115 

Faraday's  law,  123 
Fluorine,  100 
Fluorite,  100 
Formulae,  140,  197 
Frankland,  210 
Freezing  point,  59, 178 

Gallium,  241 
Gaseous  state,  49 
Gases,  Different,  65 
Gay  Lussac's  law,  151 
Geochemistry,  30 
Germanium,  241 
Gold  extraction,  44 
Gradation  of  properties,  104 
Grain-molecules,  200 
Granite,  94 
Grape  sugar,  225 
Graphic  formulae,  213 
Graphite,  217 

Haematite,  29 

Haloid  salts,  109 

Halogens,  100 

Heat  of  chemical  combination,  48 

of  formation,  246 

,  Expansion  by,  19 

Helium,  238,  243 

Helmholtz,  209 

Homogeneity  of  chemical  change, 

Hydrogen,  82,  112 

• chloride,  154 

sulphide,  207 

Inert  gases,  238 
Iodine,  100 
Ionic  dissociation,  180 
Iodine  and  iron,  38,  172 
Iron,  oxides,  71,  172 
Iron  rust,  24 


Iron  scale,  23 
Isomerism,  222 

Kekule,  210 
Kinetic  theory,  51 

Lactic  acid,  225 
Lavoisier,  78 
Le  Bel,  223 

Lecoq  de  Boisbaudran,  241 
Light,  refraction  of,  19 
Lime,  39,  102 
Limonite,  29 
Liquefaction  of  gases,  60 
Lockyer,  172 
Lothar  Meyer,  229 

Magnesium,  83 

Magnetite,  29 

Mass  action,  247 

Mass,  Conservation  of,  76 

Matter,  Discrete  structure,  58 

Melting-point,  59,  178 

Mendel6eff,  229,  241 

Mercury,  99,  175 

Metallic  elements,  99,  106 

Methane,  206 

Metric  system,  68 

Mixture,  Mechanical,  41 

Moissan,  104 

Molecules,  141,  159 

Molecular  compounds,  215 

volumes,  166 

weights,  143,  161 

Monatomic  molecules,  168 
Monazite  sand,  233 
Multiple  proportions,  127 
equivalence,  123 

Newlands,  229 
Nilson,  241 
Niton,  243 
Nitrogen,  37 

dioxide,  169 

Non-metals,  106 

Onnes,  147 
Optical  activity,  225 
Organic  chemistry,  218 
Ores,  Metallic,  43 
Oxides,  71,  108 
Oxygen,  27 
Oxy-salts,  109 
Ozone,  216 

Partial  valency,  216 
Particles,  Visualized,  50 
Perfumes  from  tar,  43 


INDEX 


255 


Periodic  classification,  230 
Pewter,  107 
Physical  change,  22 
Phosphorus,  167 
Preferential  action,  35,  37,  83 
Purity,  Chemical,  188 
Pyrophoric  iron,  49 

Quantitative  transformation,  67 
Quicklime,  102 

Radioactivity,  95,  136,  243 
Radium,  98,  243 
Ramsay  and  Soddy,  135 
Raoult,  179 
Rare  earths,  233 
Reciprocal  equivalence,  123 
Residual  valency,  216 
Reversible  reaction,  247 
Rusting  of  iron,  25 
Rutherford,  135 

Salt,  Common,  100 
Salts,  108 
Scandium,  241 
Silicon,  113 
Sodium,  101 

oxides,  125,  144 

Solder,  107 
Solution,  85 
Specific  heat,  173 
Spectroscopy,  20 
Steel,  107 
Stereochemistry,  222 


Structure,  Chemical,  210 

Sugar,  177 

Sulphur,  combination  with  iron,  39 

oxides,  128 

,  vapour  density  of,  167 

Symbols,  137 

Synthetical  chemistry,  33,  93 

Tartaric  acid,  225 
Tautomerism,  222 
Thermochemistry,  244 
Thomson,  133 
Thorium,  233,  243 
Transmutation^  .26,  90,  243 
Transition  phases,  75 

Uranium,  243 

Valency,  204 

Van't  HotF,  223 

Velocity  of  chemical  change,  73 

Vitalism,  219,  226 

Volumelrio  analysis,  203 

combination,  147 

Water,  Composition  of,  80 
— — ,  Formula  of,  143 

vapour  in  air,  53 

,  volumetric  composition,  88, 

151 

Weights  and  measures,  68 
Winkler,  241 

Zero,  Absolute,  143 
Zinc,  83 


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26.  The  Dawn  of  History. 

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Ireland.  A  study  of  the  geology  and  physical  geography  in  connec- 
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13.  Medieval  Europe. 

By  H.  W.  C.  DAVIS,  Fellow  at  Balliol  College,  Oxford,  author  of 
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33.  The  History  of  England. 

By  A.  F.  POLLARD,  Professor  of  English  History,  University  of 
London.  "Professor  Pollard  is  to  be  ranked  among  the  few  leading 
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3.  The  French  Revolution. 

By  HILAIRE  BELLOC.  "For  the  busy  man  it  would  be  difficult  to 
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By  G.  H.  FERRIS,  author  of  Riissia  in  Revolution,  etc.  The  Hon. 
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22.  The  Papacy  and  Modern  Times. 

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8.  Polar  Exploration. 

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18.  The  Opening-up  of  Africa. 

By  Sir  H.  H.  JOHNSTON.  The  first  living  authority  on  the  subject 
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49.  Elements  of  Political  Economy. 

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Faculty  of  Commerce  and  Administration,  University  of  Manchester. 
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structure  and  working  of  the  modern  business  world. 

1.  Parliament.     Its   History,    Constitution,    and 
Practice. 

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"Can  be  praised  without  reserve.  Admirably  clear." — New  York  Sun. 

16.  Liberalism. 

By  PROF.  L.  T.  HOBHOUSE,  author  of  Democracy  and  Reaction.  A 
masterly  philosophical  and  historical  review  of  the  subject. 

5.  The  Stock  Exchange. 

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other  terms  which  the  title  suggests. 

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By  J.  RAMSAY  MACDONALD,  Chairman  of  the  British  Labor  Party. 
"The  latest  authoritative  exposition  of  Socialism." — San  Francisco 
Argonaut. 

28.  The  Evolution  of  Industry. 

By  D.  H.  MACGREGOR,  Professor  of  Political  Economy,  University 
of  Leeds.  An  outline  of  the  recent  changes  that  have  given  us  the 
present  conditions  of  the  working  classes  and  the  principles  involved. 

29.  Elements  of  English  Law. 

By  W.  M.  GELDART,  Vinerian  Professor  of  English  Law,  Oxford.  A 
simple  statement  of  the  basic  principles  of  the  English  legal  system 
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6.  Irish  Nationality. 

By  MRS.  J.  R.  GREEN.  A  brilliant  account  of  the  genius  and  mission 
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68.  Disease  and  Its  Causes. 

By  W.  T.  COUNCILMAN,  M.  D.,  LL.  D.,  Professor  of  Pathology,  Har- 
vard University. 

85.  Sex. 

By  J.  ARTHUR  THOMSON  and  PATRICK  GEDDES,  joint  authors  of  The 
Evolution  of  Sex. 

71.  Plant  Life. 

By  J.  B.  FARMER,  D.  Sc.,  F.  R.  S.,  Professor  of  Botany  in  the  Impe- 
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view  of  function. 

63.  The  Origin  and  Nature  of  Life. 

By  BENJAMIN  M.  MOORE,  Professor  of  Bio  Chemistry,  Liverpool. 
Perhaps  the  chapters  on  "The  Origin  of  Life"  and  "How  Life  Came 
to  Earth"  will  attract  most  attention,  as  throwing  the  newest  light 
upon  matters  of  very  ancient  controversy. 

53.  Electricity. 

By  GISBERT  KAPP,  Professor  of  Electrical  Engineering,  University  of 
Birmingham. 

54.  The  Making  of  the  Earth. 

By  J.  W.  GREGORY,  Professor  of  Geology,  Glasgow  University.  38 
maps  and  figures.  Describes  the  origin  of  the  earth,  the  formation 
and  changes  of  its  surface  and  structure,  its  geological  history,  the 
first  appearance  of  life,  and  its  influence  upon  the  globe. 

56.  Man:  A  History  of  the  Human  Body. 

By  A.  KEITH,  M.  D.,  Hunterian  Professor,  Royal  College  of  Sur- 
geons. Shows  how  the  human  body  developed. 

74.  Nerves. 

By  DAVID  FRASER  HARRIS,  M.  D.,  Professor  of  Physiology,  Dalhousie 
University,  Halifax.  Explains  in  non-technical  language  the  place 
and  powers  of  the  nervous  system,  more  particularly  of  those  regions 
of  the  system  whose  activities  are  not  associated  with  the  rousing  of 
consciousness. 

21.  An  Introduction  to  Science. 

By  PROF.  J.  ARTHUR  THOMSON,  Science  Editor  of  the  Home  Univer- 
sity Library.  For  those  unacquainted  with  the  scientific  volumes  in 
the  series,  this  would  prove  an  excellent  introduction. 

14.  Evolution. 

By  PROF.  J.  ARTHUR  THOMSON  and  PROF.  PATRICK  GEDDES.  Explains 
to  the  layman  what  the  title  means  to  the  scientific  world. 

23.  Astronomy. 

By  A.  R.  HINKS,  Chief  Assistant  at  the  Cambridge  Observatory. 
"Decidedly  original  in  substance,  and  the  most  readable  and  informa- 
tive little  book  on  modern  astronomy  we  have  seen  for  a  long  time." 

— Nature. 

24.  Psychical  Research. 

By  PROF.  W.  F.  BARRETT,  formerly  President  of  the  Society  for 
Psychical  Research.  A  strictly  scientific  examination. 


9.  The  Evolution  of  Plants. 

By  DR.  D.  H.  SCOTT,  President  of  the  Linnean  Society  of  London. 
The  story  of  the  development  of  flowering  plants,  from  the  earliest 
zoological  times,  unlocked  from  technical  language. 

43.  Matter  and  Energy. 

By  F.  SODDY,  Lecturer  in  Physical  Chemistry  and  Radioactivity, 
University  of  Glasgow.  "Brilliant.  Can  hardly  be  surpassed.  Sure 
to  attract  attention." — New  York  Sun. 

41.  Psychology,  The  Study  of  Behaviour. 

By  WILLIAM  McDoUGALL,  of  Oxford.  A  well  digested  summary  of 
the  essentials  of  the  science  put  in  excellent  literary  form  by  a  lead- 
ing authority. 

42.  The  Principles  of  Physiology. 

By  PROF.  J.  G.  MCKENDRICK.  A  compact  statement  by  the  Emeritus 
Professor  at  Glasgow,  for  uninstructed  readers. 

37.  Anthropology. 

By  R.  R.  MARETT,  Reader  in  Social  Anthropology,  Oxford.  Seeks  to 
plot  out  and  sum  up  the  general  series  of  changes,  bodily  and  mental, 
undergone  by  man  in  the  course  of  history.  "Excellent.  So  enthusi- 
astic, so  clear  and  witty,  and  so  well  adapted  to  the  general  reader." 
— American  Library  Association  Booklist. 

17.  Crime  and  Insanity. 

By  DR.  C.  A.   MERCIER,  author  of  Text-Book  of  Insanity,  etc. 

12.  The  Animal  World. 

By  PROF.  F.  W.  GAMBLE. 

15.  Introduction  to  Mathematics. 

By  A.  N.  WHITEIIEAD,  author  of  Universal  Algebra. 

PHILOSOPHY  AND  RELIGION 
69.  A  History  of  Freedom  of  Thought. 

By  JOHN  B.  BURY,  M.  A.,  LL.  D.,  Regius  Professor  of  Modern  His- 
tory in  Cambridge  University.  Summarizes  the  history  of  the  long 
struggle  between  authority  and  reason  and  of  the  emergence  of  the 
principle  that  coercion  of  opinion  is  a  mistake. 

55.  Missions :  The^r  Rise  and  Development. 

By  MRS.  MANDELL  CREIGHTON,  author  of  History  of  England.  The 
author  seeks  to  prove  that  missions  have  done  more  to  civilize  the 
world  than  any  other  human  agency. 

52.  Ethics. 

By  G.  E.  MOORE,  Lecturer  in  Moral  Science,  Cambridge.  Discusses 
what  is  right  and  what  is  wrong,  and  the  whys  and  wherefores. 

65.  The  Literature  of  the  Old  Testament. 

By  GEORGE  F.  MOORE,  Professor  of  the  History  of  Religipn,  Harvard 
University.  "A  popular  work  of  the  highest  order.  Will  be  profit- 
able to  anybody  who  cares  enough  about  Bible  study  to  read  a  serious 
book  on  the  subject." — American  Journal  of  Theology. 

50.  The  Making  of  the  New  Testament. 

By  B.  W.  BACON,  Professor  of  New  Testament  Criticism,  Yale.  An 
authoritative  summary  of  the  results  of  modern  critical  research 
with  regard  to  the  origins  of  the  New  Testament. 


35.  The  Problems  of  Philosophy. 

By  BERTRAND  RUSSELL,  Lecturer  and  Late  Fellow,  Trinity  College, 
Cambridge. 

44.  Buddhism. 

By  MRS.  RHYS  DAVIDS,  Lecturer  on  Indian  Philosophy,  Manchester. 
A  review  of  that  religion  and  body  of  culture  which  is  to  a  large 
part  of  the  human  race,  chiefly  situated  in  Southern  Asia,  what 
Christianity  is  to  us  of  the  West. 

46.  English  Sects:  A  History  of  Nonconformity. 

By  W.  B.  SELBIE,  Principal  of  Manchester  College,  Oxford. 

60.  Comparative  Religion. 

By  PROF.  J.  ESTLIN  CARPENTER.  "One  of  the  few  authorities  on  this 
subject  compares  all  the  religions  to  see  what  they  have  to  offer  on 
the  great  themes  of  religion." — Christian  Work  and  Evangelist. 

LITERATURE  AND  ART 
73.  Euripides  and  His  Age. 

By  GILBERT  MURRAY,  Regius  Professor  of  Greek,  Oxford.  Brings 
before  the  reader  an  undisputedly  great  poet  and  thinker,  an  amaz- 
ingly successful  playwright,  and  a  figure  of  high  significance  in  the 
history  of  humanity. 

81.  Chaucer  and  His  Times. 

By  GRACE  E.  HADOW,  Lecturer  Lady  Margaret  Hall,  Oxford;  Late 
Reader,  Bryn  Mawr. 

70.  Ancient  Art  and  Ritual. 

By  JANE  E.  HARRISON,  LL.  D.,  D.  Litt.  "One  of  the  100  most  impor- 
tant books  of  1913." — New  York  Times  Review. 

61.  The  Victorian  Age  in  Literature. 

By  G.  K.  CHESTERTON.  The  most  powerfully  sustained  and  brilliant 
piece  of  writing  Mr.  Chesterton  has  yet  published. 

59.  Dr.  Johnson  and  His  Circle. 

By  JOHN  BAILEY.  Johnson's  life,  character,  works,  and  friendships 
are  surveyed;  and  there  is  a  notable  vindication  of  the  "Genius  of 
Boswell." 

58.  The  Newspaper. 

By  G.  BINNEY  DIBBLEE.  The  first  full  account,  from  the  inside,  of 
newspaper  organization  as  its  exists  to-day. 

62.  Painters  and  Painting. 

By  SIR  FREDERICK  WEDMORE.     With  16  half-tone  illustrations. 

64.  The  Literature  of  Germany. 

By  J.  G.  ROBERTSON. 

48.  Great  Writers  of  America. 

By  W.  P.  TRENT  and  JOHN  ERSKINE,  of  Columbia  University.  Gives 
the  essential  facts  as  to  the  lives  and  works  of  Franklin,  Washington 
Irving,  Bryant,  Cooper,  Hawthorne,  Poe,  Emerson,  and  the  other 
Transcendentalists,  Oliver  Wendell  Holmes  and  the  other  New  Eng- 
land poets.  Motley  and  the  other  historians,  Webster  and  Abraham 
Lincoln,  Mrs.  Stowe,  Walt  Whitman,  Bret  Harte,  and  Mark  Twain. 


40.  The  English  Language. 

By  L.  P.  SMITH.  A  concise  history  of  the  origin  and  development 
of  the  English  language.  "Has  certainly  managed  to  include  a  vast 
amount  of  information,  and,  while  his  writing  is  clear  and  lucid,  he 
is  always  in  touch  with  life." — The  Athenaeum. 

45.  Medieval  English  Literature. 

By  W.  P.  KER,  Professor  of  English  Literature,  University  College, 
London.  "One  of  the  soundest  scholars.  His  style  is  effective,  sim- 
ple, yet  never  dry." — The  Athenaeum. 

27.  Modern  English  Literature. 

By  G.  H.  MAIR.  From  Wyatt  and  Surrey  to  Synge  and  Yeats.  "A 
most  suggestive  book,  one  of  the  best  of  this  great  series." — Chicago 
Evening  Post. 

2.  Shakespeare. 

By  JOHN  MASEFIELD.  "One  of  the  very  few  indispensable  adjuncts 
to  a  Shakespearean  Library." — Boston  Transcript. 

31.  Landmarks  in  French  Literature. 

By  G.  L.  STRACHEY,  Scholar  of  Trinity  College,  Cambridge.  "For  a 
survey  of  the  oustanding  figures  of  French  literature  with  an  acute 
analysis  of  the  contribution  which  each  made  to  his  time  and  to  the 
general  mass  there  has  been  no  book  as  yet  published  so  judicially 
interesting." — The  Chautauquan. 

38.  Architecture. 

By  PROF.  W.  R.  LETHABY.  An  introduction  to  the  history  and 
theory  of  the  art  of  building.  "Professor  Lethaby's  scholarship  and 
extraordinary  knowledge  of  the  most  recent  discoveries  of  archaeo- 
logical research  provide  the  reader  with  a  new  outlook  and  with  new 
facts." — The  Athenaeum. 

66.  Writing  English  Prose. 

By  WILLIAM  T.  BREWSTER,  Professor  of  English,  Columbia  Univer- 
sity. "Should  be  put  into  the  hands  of  every  man  who  is  beginning 
to  write  and  of  every  teacher  of  English  that  has  brains  enough  to 
understand  sense." — New  York  Sun. 

83.  William  Morris:  His  Work  and  Influence. 

By  A.  GLUTTON  BROCK,  author  of  Shellev:  The  Man  and  the  Poet. 
William  Morris  believed  that  the  artist  should  toil  for  love  of  his 
work  rather  than  the  gain  of  his  employer,  and  so  he  turned  from 
making  works  of  art  to  remaking  society. 

OTHER   VOLUMES  IN  PREPARATION. 

HENRY  HOLT  AND  COMPANY 
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